Light-emitting element, light-emitting device, and electronic device

ABSTRACT

A light-emitting element disclosed in the present invention includes a light-emitting layer and a first layer between a first electrode and a second electrode, in which the first layer is provided between the light-emitting layer and the first electrode. The present invention is characterized by the device structure in which the first layer comprising a hole-transporting material is doped with a hole-blocking material or an organic compound having a large dipole moment. This structure allows the formation of a high performance light-emitting element with high luminous efficiency and long lifetime. The device structure of the present invention facilitates the control of the rate of the carrier transport, and thus, leads to the formation of a light-emitting element with a well-controlled carrier balance, which contributes to the excellent characteristics of the light-emitting element of the present invention.

TECHNICAL FIELD

The present invention relates to current excitation type light-emittingelements. Further, the present invention relates to light-emittingdevices and electronic devices which have the light-emitting element.

BACKGROUND ART

In recent years, research and development have been extensivelyconducted on light-emitting elements utilizing electroluminescence. In abasic structure of such a light-emitting element, a light-emissivesubstance is interposed between a pair of electrodes. By voltageapplication to this element, light emission can be obtained from thelight-emissive substance.

Since such a light-emitting element is of self-light-emitting type, itis considered that the light-emitting element has advantages over aliquid crystal display in that visibility of pixels is high, backlightis not required, and so on and is therefore suitable as flat paneldisplay elements. Further, other advantages of such a light-emittingelement are that the element can be manufactured to be thin andlightweight and the response speed is very high.

Since the light-emitting element can be formed into a film shape, lightemission can be easily obtained from a flat surface with large-area.This is a feature which is difficult to be obtained by point lightsources typified by an incandescent lamp and an LED or linear lightsources typified by a fluorescent lamp. Accordingly, the light-emittingelement is extremely effective for use as a flat light source applicableto illumination and the like.

Light-emitting elements utilizing electroluminescence are classifiedbroadly according to whether they use an organic compound or aninorganic compound as a light-emissive substance.

When an organic compound is used as a light-emitting substance,electrons and holes are each injected into a layer including alight-emitting organic compound from a pair of electrodes by voltageapplication to a light-emitting element, so that a current flowstherethrough. The electrons and holes (carriers) are recombined, andthus, the light-emitting organic compound is excited. The light-emittingorganic compound relaxes to a ground state from the excited state,thereby emitting light. Based on this mechanism, such a light-emittingelement is referred to as current excitation type light-emittingelement.

Note that the excited state of an organic compound can be a singletexcited state or a triplet excited state, and luminescence from thesinglet excited state is referred to as fluorescence, and luminescencefrom the triplet excited state is referred to as phosphorescence.

In improving element characteristics of such a light-emitting element,there are a number of problems which depend on a material used, and inorder to solve the problems, improvement of an element structure,development of a material, and the like have been carried out.

For example, in Non-patent Document 1: Tetsuo TSUTSUI, and eight others,Japanese Journal of Applied Physics, Vol. 38, L1502-L1504 (1999), ahole-blocking layer is provided, whereby light is efficiently emittedfrom a light-emitting element using a phosphorescent material. However,as described in Non-patent Document 1, the hole-blocking layer does nothave durability and a lifetime of the light-emitting element isextremely short. Thus, it has been desired to develop a light-emittingelement of which light-emitting efficiency is high and lifetime is long.

DISCLOSURE OF INVENTION

In view of the above problem, it is an object of the present inventionto provide a light-emitting element with high luminous efficiency, highluminous efficiency, and a long lifetime. The object of the presentinvention also includes provision of a light-emitting device and anelectronic device with high luminous efficiency, high luminousefficiency, and a long lifetime.

As a result of diligent study, the present inventors found that alight-emitting element with high luminous efficiency can be obtained byproviding a layer for controlling the carrier transport.

Thus, according to one feature of the present invention, alight-emitting element includes a light-emitting layer and a first layerbetween a first electrode and a second electrode, in which the firstlayer is provided between the light-emitting layer and the firstelectrode. The first layer contains a first organic compound and asecond organic compound, and a weight percent of the first organiccompound is higher than that of the second organic compound. The firstorganic compound has a hole-transporting property, and the secondorganic compound is a substance into which a hole is not injected andwhich reduces a hole-transporting property of the first layer. Lightemission is obtained from the light-emitting layer by applying voltageto the first electrode and the second electrode so that a potential ofthe first electrode is higher than that of the second electrode.

According to another feature of the present invention, a light-emittingelement includes a light-emitting layer and a first layer between afirst electrode and a second electrode, in which the first layer isprovided between the light-emitting layer and the first electrode. Thefirst layer contains a first organic compound and a second organiccompound, and a weight percent of the first organic compound is higherthan that of the second organic compound. According to the feature ofthe present invention, the first organic compound has ahole-transporting property, while the second organic compound is ahole-blocking material having a dipole moment of larger than or equal to2.0 debye. Light emission is obtained from the light-emitting layer byapplying voltage to the first electrode and the second electrode so thata potential of the first electrode is higher than a potential of thesecond electrode.

According to another feature of the present invention, a light-emittingelement having the following structure is provided. Namely, thelight-emitting element of the present invention includes alight-emitting layer and a first layer between a first electrode and asecond electrode, in which the first layer is provided between thelight-emitting layer and the first electrode. The first layer contains afirst organic compound and a second organic compound, and a weightpercent of the first organic compound is higher than that of the secondorganic compound. Here, the first organic compound has ahole-transporting property and a difference in the ionization potentialbetween the second organic compound and the first organic compound isgreater than or equal to 0.5 eV. The dipole moment of the second organiccompound is also larger than or equal to 2.0 debye. Light emission isobtained from the light-emitting layer by applying voltage to the firstelectrode and the second electrode so that a potential of the firstelectrode is higher than a potential of the second electrode.

According to another feature of the present invention, a light-emittingelement includes a light-emitting layer and a first layer between afirst electrode and a second electrode, in which the first layer isprovided between the light-emitting layer and the first electrode. Thefirst layer contains a first organic compound and a second organiccompound, and a weight percent of the first organic compound is higherthan that of the second organic compound. According to this feature ofthe present invention, the first organic compound has ahole-transporting property, and the second organic compound has anionization potential of greater than or equal to 5.8 eV andsimultaneously has a dipole moment of larger than or equal to 2.0 debye.Light emission is obtained from the light-emitting layer by applyingvoltage to the first electrode and the second electrode so that apotential of the first electrode is higher than a potential of thesecond electrode.

Another feature of the present invention provides a light-emittingelement having the following structure. Specifically, the light-emittingelement of the present invention includes a light-emitting layer and afirst layer between a first electrode and a second electrode, in whichthe first layer is provided between the light-emitting layer and thefirst electrode. The first layer contains a first organic compound and asecond organic compound, and a weight percent of the first organiccompound is higher than that of the second organic compound. The firstorganic compound has a hole-transporting property, and the difference inthe ionization potential between the second organic compound and thefirst organic compound is greater than or equal to 0.5 eV. Here, thesecond organic compound has a heterocycle. Light emission is obtainedfrom the light-emitting layer by applying voltage to the first electrodeand the second electrode so that a potential of the first electrode ishigher than a potential of the second electrode.

A light-emitting having the following structure is also included in thepresent invention. Namely, the present invention provides alight-emitting element having a light-emitting layer and a first layerbetween a first electrode and a second electrode, in which the firstlayer is provided between the light-emitting layer and the firstelectrode. The first layer contains a first organic compound and asecond organic compound, and a weight percent of the first organiccompound is higher than that of the second organic compound. The firstorganic compound has a hole-transporting property, and the secondorganic compound has an ionization potential of greater than or equal to5.8 eV and also has a heterocycle. Light emission is obtained from thelight-emitting layer by applying voltage to the first electrode and thesecond electrode so that a potential of the first electrode is higherthan a potential of the second electrode.

According to another feature of the present invention, a light-emittingelement includes a light-emitting layer and a first layer between afirst electrode and a second electrode, in which the first layer isprovided between the light-emitting layer and the first electrode. Thefirst layer contains a first organic compound and a second organiccompound, and a weight percent of the first organic compound is higherthan that of the second organic compound. The first organic compound hasa hole-transporting property, and the second organic compound is any ofan oxadiazole derivative, a triazole derivative, and phenanthrolinederivative. Light emission is obtained from the light-emitting layer byapplying voltage to the first electrode and the second electrode so thata potential of the first electrode is higher than a potential of thesecond electrode.

Another feature of the present invention permits providing alight-emitting element comprising a light-emitting layer and a firstlayer between a first electrode and a second electrode, in which thefirst layer is provided between the light-emitting layer and the firstelectrode. The first layer contains a first organic compound and asecond organic compound, and a weight percent of the first organiccompound is higher than that of the second organic compound. The firstorganic compound has a hole-transporting property, while the secondorganic compound is any of1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene,3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole, andbathocuproine. Light emission is obtained from the light-emitting layerby applying voltage to the first electrode and the second electrode sothat a potential of the first electrode is higher than a potential ofthe second electrode.

In any of the above structures, the light-emitting element has a featurethat a concentration of the second organic compound in the first layeris greater than or equal to 1 wt % and less than or equal to 20 wt %.This concentration range allows the formation of a light-emittingelement with a long lifetime.

Further, in any of the above structures, the first layer may not be incontact with the first electrode and the light-emitting layer. In otherwords, a layer may be provided between the first layer and thelight-emitting layer and another layer may be provided between the firstlayer and the first electrode.

Still further, in any of the above structures, a thickness of the firstlayer is preferably greater than or equal to 1 nm and less than or equalto 20 nm.

Furthermore, in any of the above structures, the first layer and thelight-emitting layer may be provided so as to be in contact with eachother.

In addition, the present invention includes a light-emitting devicehaving the above light-emitting element. The light-emitting device shownin this specification includes an image display device, a light-emittingdevice, or a light source (including a lighting device). Further, thelight-emitting device of the present invention includes all thefollowing modules: a module in which a connector such as a flexibleprinted circuit (FPC), a tape automated bonding (TAB) tape, or a tapecarrier package (TCP) is attached to a panel connector provided with alight-emitting element; a module provided with a printed wiring board atthe end of the TAB tape or the TCP; and a module in which an integratedcircuit (IC) is directly mounted by a chip on glass (COG) method to asubstrate on which a light-emitting element is formed.

Furthermore, the present invention includes an electronic device whichis equipped with the light-emitting element of the present invention forthe display portion. Therefore, one feature of the electronic device ofthe present invention is to include a display portion having both theabove light-emitting element and a controller which controls lightemission of the light-emitting element.

In a light-emitting element of the present invention, a layer forcontrolling the carrier transport is provided; thus, a light-emittingelement with high luminous efficiency can be obtained. In addition, alight-emitting element with high luminous efficiency and a long lifetimecan be obtained.

Further, the light-emitting element of the present invention is appliedto a light-emitting device and an electronic device, whereby alight-emitting device and an electronic device with high luminousefficiency and reduced power consumption can be obtained. In addition, alight-emitting device and an electronic device which consumes low powerand have a long lifetime can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are views each illustrating a light-emitting element ofthe present invention;

FIGS. 2A and 2B are views each illustrating a light-emitting element ofthe present invention;

FIGS. 3A to 3C are views each illustrating a light-emitting element ofthe present invention;

FIG. 4 is a view illustrating a light-emitting element of the presentinvention;

FIG. 5 is a view illustrating a light-emitting element of the presentinvention;

FIGS. 6A and 6B are views illustrating a light-emitting device of thepresent invention;

FIGS. 7A and 7B are views illustrating a light-emitting device of thepresent invention;

FIGS. 8A to 8D are views each illustrating an electronic device of thepresent invention;

FIG. 9 is a view illustrating an electronic device of the presentinvention;

FIG. 10 is a view illustrating an electronic device of the presentinvention;

FIG. 11 is a view illustrating an electronic device of the presentinvention;

FIG. 12 is a view illustrating a lighting device of the presentinvention;

FIG. 13 is a view illustrating a lighting device of the presentinvention;

FIG. 14 is a view illustrating a light-emitting element of embodiments;

FIG. 15 is a graph illustrating current density-luminancecharacteristics of the light-emitting elements manufactured inEmbodiment 1;

FIG. 16 is a graph illustrating voltage-luminance characteristics of thelight-emitting elements manufactured in Embodiment 1;

FIG. 17 is a graph illustrating luminance-current efficiencycharacteristics of the light-emitting elements manufactured inEmbodiment 1;

FIG. 18 is a graph illustrating the emission spectra of thelight-emitting elements manufactured in Embodiment 1;

FIG. 19 is a graph illustrating the results of the continuous lightingtests obtained by constant current driving of the light-emittingelements manufactured in Embodiment 1;

FIG. 20 is a view illustrating a light-emitting element of embodiments;

FIG. 21 is a graph illustrating current density-luminancecharacteristics of the light-emitting elements manufactured inEmbodiment 2;

FIG. 22 is a graph illustrating voltage-luminance characteristics of thelight-emitting elements manufactured in Embodiment 2;

FIG. 23 is a graph illustrating luminance-current efficiencycharacteristics of the light-emitting elements manufactured inEmbodiment 2;

FIG. 24 is a graph illustrating the emission spectra of thelight-emitting elements manufactured in Embodiment 2;

FIG. 25 is a graph illustrating current density-luminancecharacteristics of the light-emitting elements manufactured inEmbodiment 3;

FIG. 26 is a graph illustrating voltage-luminance characteristics of thelight-emitting elements manufactured in Embodiment 3;

FIG. 27 is a graph illustrating luminance-current efficiencycharacteristics of the light-emitting elements manufactured inEmbodiment 3;

FIG. 28 is a graph illustrating the emission spectra of thelight-emitting elements manufactured in Embodiment 3;

FIG. 29 is a graph illustrating current density-luminancecharacteristics of the light-emitting elements manufactured inEmbodiment 4;

FIG. 30 is a graph illustrating voltage-luminance characteristics of thelight-emitting elements manufactured in Embodiment 4;

FIG. 31 is a graph illustrating luminance-current efficiencycharacteristics of the light-emitting elements manufactured inEmbodiment 4;

FIG. 32 is a graph illustrating the emission spectra of thelight-emitting elements manufactured in Embodiment 4;

FIG. 33 is a graph illustrating current density-luminancecharacteristics of the light-emitting elements manufactured inEmbodiment 5;

FIG. 34 is a graph illustrating voltage-luminance characteristics of thelight-emitting elements manufactured in Embodiment 5;

FIG. 35 is a graph illustrating luminance-current efficiencycharacteristics of the light-emitting elements manufactured inEmbodiment 5;

FIG. 36 is a graph illustrating the emission spectra of thelight-emitting elements manufactured in Embodiment 5;

FIG. 37 is a graph illustrating oxidation characteristics of NPB;

FIG. 38 is a graph illustrating oxidation characteristics of OXD-7;

FIG. 39 is a graph illustrating oxidation characteristics of TAZOL; and

FIG. 40 is a graph illustrating oxidation characteristics of BCP.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment modes and embodiments of the present invention will bedescribed hereinafter with reference to the accompanying drawings.However, the present invention is not limited to the followingdescription, and those skilled in the art can easily understand thatmodes and details of the present invention can be changed in variousways without departing from the purpose and the scope of the presentinvention. Therefore, the present invention should not be interpreted asbeing limited to the description of the embodiment modes and embodimentsbelow.

Note that, in this specification, the word “composite” refers not onlyto a state in which two materials are simply mixed but also to a statein which a plurality of materials are mixed and charges are transferredbetween the materials.

EMBODIMENT MODE 1

One mode of a light-emitting element of the present invention will bedescribed with reference to FIG. 1A. This embodiment mode will describea light-emitting element in which a layer for controlling the transportof holes is provided as a layer for controlling the carrier transport.

The light-emitting element of the present invention has a plurality oflayers between a pair of electrodes. The plurality of layers is stackedby combining layers formed of a substance having a highcarrier-injecting property and a substance having a highcarrier-transporting property so that a light-emitting region is formedat a position away from the electrodes, that is, so that carriers arerecombined at a position away from the electrodes.

In this embodiment mode, a light-emitting element includes a firstelectrode 202, a second electrode 204, and an EL layer 203 providedbetween the first electrode 202 and the second electrode 204. Note thatdescription will be made on the assumption that the first electrode 202functions as an anode and the second electrode 204 functions as acathode, in this embodiment mode. That is, light emission is obtainedwhen a voltage is applied to the first electrode 202 and the secondelectrode 204 so that the potential of the first electrode 202 is higherthan the potential of the second electrode 204.

A substrate 201 is used as a support of the light-emitting element. Asthe substrate 201, glass, plastic, or the like can be used, for example.Note that materials other than glass or plastic can be used as long asthey can function as a support of a light-emitting element.

The first electrode 202 is preferably formed using a material with ahigh work function (specifically, 4.0 eV or more) such as metals,alloys, electrically conductive compounds, or a mixture of them.Specifically, indium tin oxide (ITO), ITO containing silicon or siliconoxide, indium zinc oxide (IZO), indium oxide containing tungsten oxideand zinc oxide (IWZO), and the like can be given. Such conductive metaloxide films are generally formed by a sputtering method, but may be alsoformed by an ink-jet method, a spin coating method, or the like byapplication of a sol-gel method or the like. For example, indium zincoxide (IZO) can be deposited by a sputtering method using a target inwhich 1 to 20 wt % of zinc oxide is added to indium oxide. Further,indium oxide containing tungsten oxide and zinc oxide (IWZO) can bedeposited by a sputtering method using a target in which 0.5 to 5 wt %of tungsten oxide and 0.1 to 1 wt % of zinc oxide are added to indiumoxide. Besides, gold (Au), platinum (Pt), nickel (Ni), tungsten (W),chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu),palladium (Pd), titanium (Ti), nitride of metal materials (for example,titanium nitride), and the like can be given.

When a layer containing a composite material described below is used asa layer in contact with the first electrode, the first electrode can beformed using various metals, alloys, electrically conductive compound, amixture of them, or the like regardless of their work functions. Forexample, aluminum (Al), silver (Ag), an aluminum alloy (e.g., AlSi), orthe like can be used. Besides, an element belonging to Group 1 or 2 ofthe periodic table which has a low work function, that is, alkali metalssuch as lithium (Li) and cesium (Cs), alkaline earth metals such asmagnesium (Mg), calcium (Ca), and strontium (Sr), alloys of them (forexample, MgAg and AlLi) can be used. Moreover, rare earth metals such aseuropium (Eu) and ytterbium (Yb); alloys of them; or the like can bealso used. A film made of an alkali metal, an alkaline earth metal, oran alloy of them can be formed by a vacuum deposition method. Further, afilm made of an alloy of an alkali metal or an alkaline earth metal canbe also formed by a sputtering method. It is also possible to deposit asilver paste or the like by an ink-jet method or the like.

The EL layer 203 shown in this embodiment mode includes a hole-injectinglayer 211, a layer 212 for controlling the carrier transport, ahole-transporting layer 213, a light-emitting layer 214, anelectron-transporting layer 215, and an electron-injecting layer 216.Note that the structure of the EL layer 203 is not limited to the abovestructure as long as at least the layer for controlling the carriertransport 212 shown in this embodiment and the light-emitting layer 214are included. That is, the structure of the EL layer 203 is notparticularly limited. Thus, layers which contain a substance having ahigh electron-transporting property, a substance having a highhole-transporting property, a substance having a high electron-injectingproperty, a substance having a high hole-injecting property, a substancehaving a bipolar property (a substance with a high electron-transportingproperty and a high hole-transporting property), or the like may beappropriately combined with the layer for controlling the carriertransport and the light-emitting layer which are shown in thisembodiment mode to form the EL layer 203. For example, the EL layer 203can be formed in any combination of the hole-injecting layer, thehole-transporting layer, the light-emitting layer, theelectron-transporting layer, the electron-injecting layer, and the likeas long as both the layer for controlling the carrier transport 212 andthe light-emitting layer 214 are included. Specific materials forforming each layer are shown below.

The hole-injecting layer 211 is a layer containing a substance having ahigh hole-injecting property. As a substance having a highhole-injecting property, molybdenum oxide, vanadium oxide, rutheniumoxide, tungsten oxide, manganese oxide, or the like can be used.Besides, the following can be also given as organic compounds with a lowmolecular-weight: phthalocyanine-based compounds such as phthalocyanine(abbreviation: H₂PC), copper(II) phthalocyanine (abbreviation: CuPc),and vanadyl(IV) phthalocyanine (VOPc); aromatic amine compounds such as4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),4,4′-bis(N-{4-[N′-(3-methylphenyl)-N-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), and3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1); and the like.

Alternatively, the hole-injecting layer 211 can be formed using acomposite material in which a substance having an acceptor property ismixed into a substance having a high hole-transporting property. Notethat a material for forming the electrode can be selected regardless ofits work function with the use of a composite material in which asubstance having an acceptor property is mixed into a substance having ahigh hole-transporting property. That is, not only a material with ahigh work function but also a material with a low work function can beused for the first electrode 202. Such a composite material can beformed by co-depositing a substance having a high hole-transportingproperty and a substance having an acceptor property.

As an organic compound used for the composite material, variouscompounds such as aromatic amine compounds, carbazole derivatives,aromatic hydrocarbons, and compounds with a high molecular-weight (forexample, oligomer, dendrimer, or polymer) can be used. The organiccompound used for the composite material is preferably an organiccompound having a high hole-transporting property. Specifically, asubstance with a hole mobility of 10⁻⁶ cm²/Vs or more is preferablyused. However, other substances may be also used as long as thehole-transporting properties thereof are higher than theelectron-transporting properties thereof. Specific organic compoundsthat can be used for the composite material are described below.

For example, the following organic compounds can be used for thecomposite material: aromatic amine compounds such as MTDATA, TDATA,DPAB, DNTPD, DPA3B, PCzPCA1, PCzPCA2,PCzPCN1,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation:NPB or α-NPD), andN,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD); carbazole derivatives such as4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), and1,4-bis[4-(N-carbazolyl)phenyl-2,3,5,6-tetraphenylbenzene; and aromatichydrocarbon compounds such as 2-tert-butyl-9,10-di(2-naphthyl)anthracene(abbreviation: t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),9,10-bis[2-(1-naphthyl)phenyl)-2-tert-butyl-anthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene,pentacene, coronene, 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation:DPVBi), and 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene(abbreviation: DPVPA).

As a substance having an acceptor property, organic compounds such as7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ) and chloranil, and a transition metal oxide can be given. Inaddition, oxides of metals belonging to Groups 4 to 8 in the periodictable can be also given. Specifically, it is preferable to use vanadiumoxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide,tungsten oxide, manganese oxide, and rhenium oxide which have highelectron-accepting properties. Among them, molybdenum oxide isparticularly preferable because it is stable even in atmospheric air,has a low hygroscopic property, and is easy to handle.

As the hole-injecting layer 211, compounds with a high molecular-weight(for example, oligomer, dendrimer, or polymer) can be used. For example,the following compounds with a high molecular-weight can be used:poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine)(abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide] (abbreviation: PTPDMA),poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine (abbreviation:Poly-TPD), and the like. Further, high molecular compounds mixed withacid such as poly (3,4-ethylenedioxythiophene)/poly(styrenesulfonate)(PEDOT/PSS) and polyaniline/poly(styrenesulfonate) (PAni/PSS) can bealso used.

Further, a composite material formed by using the above polymers such asPVK, PVTPA, PTPDMA, or Poly-TPD and the above-mentioned substance havingan acceptor property can be used as the hole-injecting layer 211.

The layer 212 for controlling the carrier transport contains a firstorganic compound and a second organic compound, and a weight percent ofthe first organic compound is higher than that of the second organiccompound.

The first organic compound is a so-called hole-transporting materialwhich has a hole-transporting property higher than anelectron-transporting property. Specifically, an aromatic amine compoundcan be used. For example, MTDATA, TDATA, DPAB, DNTPD, DPA3B, PCzPCA1,PCzPCA2, PCzPCN1, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB or α-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), and the like can be given. Further, a compound witha high molecular-weight such as PVK, PVTPA, PTPDMA, or Poly-TPD can bealso used.

The second organic compound is an organic compound with a large dipolemoment to which a hole is not injected. A hole-blocking material can begiven as a substance in which a hole is not injected. In general, thehole-blocking material is a substance with a high ionization potential.In particular, a material whose ionization potential is greater than orequal to 5.8 eV is preferable because a hole is not injected to such amaterial. The ionization potential of greater than or equal to 6.0 eV isparticularly preferable. Further, a material with an ionizationpotential which is 0.5 eV greater than that of the first organiccompound is also preferable because a hole injected to the firstcompound is not transported to the second organic compound. Furthermore,the dipole moment of the second organic compound is preferably largerthan or equal to 2.0 debye. In particular, the organic compound whichhas the dipole moment of much larger than 2.0 debye can be preferablyused as the layer for controlling the carrier transport.

As the second organic compound, specifically, the following can be used:oxadiazole derivatives such as1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7) and2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD); triazole derivatives such as3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZOL) and3,5-bis(4-tert-butylphenyl)-4-phenyl-1,2,4-triazole (abbreviation:t-BuTAZ); phenanthroline derivatives such as bathophenanthroline(abbreviation: BPhen) and bathocuproine (abbreviation: BCP); or thelike.

A conceptual view of the layer for controlling the carrier transportwhich is shown in this embodiment mode is illustrated in FIG. 4. In FIG.4, a first organic compound 221 has a hole-transporting property;therefore, holes (h⁺) are readily injected to the organic compound 221and transported to the vicinal first organic compound.

On the other hand, a hole is not injected into a second organic compound222 because the ionization potential of the second organic compound 222is larger than that of the first organic compound and because theionization potential difference between the second organic compound andthe first organic compound is greater than or equal to 0.5 eV. Thus, ahole is not injected into the second organic compound and holes aretransported by hopping between the first organic compounds.

Further, the second organic compound 222 is a substance with a largedipole moment. Specifically, the dipole moment is preferably greaterthan or equal to 2.0 debye. Inclusion of the second organic compoundwith a large dipole moment decreases the transporting rate of holeswhich are transported between the first organic compounds. That is, itis considered that the second organic compound with a large dipolemoment which is located in vicinity of the first organic compound has aneffect to retard the transport of holes.

Thus, by inclusion of the second organic compound, transport rate ofholes in the layer comprising the first organic compound 221 becomeslower. In other words, addition of the second organic compound permitscontrolling the carrier transport. Further, transport rate of carrierscan be tuned by controlling the concentration of the second organiccompound.

In particular, increase in dipole moment of the second organic compoundresults in increase in the effect to reduce the transport rate of holes.Thus, in the case where a substance with a large dipole moment is usedas the second organic compound, the effect can be sufficiently obtainedeven when a weight percent of the second organic compound contained inthe layer for controlling the carrier transport is low.

As described above, control of the carrier transport allows theimprovement in carrier balance, which results in improved recombinationprobability of holes and electrons; thus, high luminous efficiency canbe obtained. Further, as shown in this embodiment mode, a structure inwhich the layer for controlling the carrier transport is providedbetween the light-emitting layer and the first electrode which functionsas an anode is particularly effective for the light-emitting element inwhich excessive holes are readily formed upon driving. This is because awell-controlled carrier balance can be achieved by introducing the layerfor controlling the carrier transport to the light-emitting elementwhich tends to exist in the hole-excessive sate, which is contributed bythe ability of the layer for controlling the carrier transport to retardthe transport of holes.

Further, by control of transport of excessive holes, the number ofcation which are formed in the light-emitting layer and vicinitiesthereof by excessive holes can be reduced. Since the cation serves as aquencher, decrease in luminous efficiency can be avoided by suppressingthe cation formation.

Since a light-emitting element using an organic compound has excessiveholes on driving in many cases, the present invention can be preferablyapplied to a number of light-emitting elements using an organiccompound.

For example, in the case of a conventional light-emitting element wherethe layer 212 for controlling the carrier transport is not provided,holes are injected into the hole-transporting layer 213 withoutreceiving the effect of the layer 212 to decelerate the transport ofholes, which allows the holes to reach the light-emitting layer 214. Ingeneral, a light-emitting element using an organic compound readilyexists in the hole-excessive state upon driving. In that case, the holeswhich cannot undergo the recombination with electrons pass through thelight-emitting layer 214. When the holes pass through the light-emittinglayer 214, the holes reach the vicinities of the interface between thelight-emitting layer 214 and the electron-transporting layer 215.Therefore, a light-emitting region is formed in the vicinities of theinterface between the light-emitting layer 214 and theelectron-transporting layer 215; thus, the recombination probability ofthe light-emitting layer 214 and luminous efficiency decrease. Further,when the holes reach the electron-transporting layer 215, theelectron-transporting layer 215 is readily deteriorated. When the numberof holes which reach the electron-transporting layer 215 is increasedover time, the recombination probability is reduced over time, whichresults in reduction in element lifetime (luminance decay over time).

In the light-emitting element of the present invention, holes injectedfrom the first electrode 202 pass through the hole-injecting layer 211,and are injected into the layer 212 for controlling the carriertransport. Transport rate of the holes injected into the layer 212 forcontrolling the carrier transport is decreased, and the hole injectionto the hole-transporting layer 213 is retarded. Therefore, the holeinjection to the light-emitting layer 214 is controlled. As a result,the light-emitting region is formed at around the center of thelight-emitting layer 214 in the light-emitting element of the presentinvention, whereas the light-emitting region is normally formed in thevicinities of the interface between the light-emitting layer 214 and theelectron-transporting layer 215 in a conventional light-emittingelement. Therefore, the possibility that the holes reach theelectron-transporting layer 215, which promotes deterioration of theelectron-transporting layer 215, is reduced.

It is important in the present invention that an organic compound havinga hole-transporting property is added with an organic compound whichreduces a hole-transporting property, instead of just applying asubstance with low hole mobility in the layer 212 for controlling thecarrier transport. Such a structure permits suppressing the change overtime of the initially well-controlled number of the injected holes, inaddition to just controlling the hole-injection into the light-emittinglayer. Accordingly, in the light-emitting element of the presentinvention, a phenomenon can be prevented, in which carrier balance isdeteriorated over time and recombination probability is decreased, whichresults in extension of element lifetime (suppression of luminance decayover time).

In the light-emitting element of the present invention, a light-emittingregion is hard to be formed at the interface between the light-emittinglayer and the hole-transporting layer or at the interface between thelight-emitting layer and the electron-transporting layer. Therefore, thelight-emitting element is hardly deteriorated because the light-emittingregion is not located in a region close to the hole-transporting layeror the electron-transporting layer. Further, change in carrier balanceover time (particularly, change over time of the number of injectedelectron) can be suppressed. Therefore, a light-emitting element withnegligible deterioration and a long lifetime can be obtained.

Note that a concentration of the second organic compound in the layerfor controlling the carrier transport is preferably greater than orequal to 1 wt % and less than or equal to 20 wt %. This concentrationrange allows the formation of the light-emitting element with alifetime. In particular, it is more preferred that the concentration ofthe second organic compound is greater than or equal to 1 wt % and lessthan or equal to 10 wt %.

The thickness of the layer for controlling the carrier transport ispreferably greater than or equal to 1 nm and less than or equal to 20nm. When the layer for controlling the carrier transport is too thick,the transport rate of carriers is excessively decreased, which resultsin high driving voltage. When the layer for controlling the carriertransport is too thin, on the other hand, it is impossible to implementthe function of controlling the carrier transport. Therefore, thethickness of the layer for controlling the carrier transport ispreferably greater than or equal to 1 nm and less than or equal to 20nm.

The hole-transporting layer 213 is a layer containing a substance havinga high hole-transporting property. As the substance having a highhole-transporting property, specifically, as a organic compound with alow molecular-weight, an aromatic amine compound such as NPB (or α-NPD),TPD, 4,4′-bis[N-(9,9-dimethylfluorene-2-yl)-N-phenylamino]biphenyl(abbreviation: DFLDPBi), or4,4′-bis[N-(spiro-9,9′-bifluorene-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB) can be used. The substances described here has amobility of 10⁻⁶ cm²/Vs or more. However, substances other than theabove substances may be also used as long as the hole-transportingproperties thereof are higher than the electron-transporting propertiesthereof. Note that the layer containing the substance having a highhole-transporting property is not limited to be a single layer but mayexist in a stacked form in which two or more layers formed of the abovesubstances are stacked.

As the hole-transporting layer 213, the compound with a highmolecular-weight such as PVK, PVTPA, PTPDMA, or Poly-TPD can be used.

The light-emitting layer 214 is a layer containing a highlylight-emissive substance, which can be formed using various materials.The compounds describe below are exemplified as organic compounds with alow molecular-weight. For example, as a light-emitting material whichexhibits bluish light, the following can be used:N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: (YGA2S);4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA), and the like. In addition, as a light-emittingmaterial which exhibits greenish light emission, the following can beused:

-   N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine    (abbreviation: 2PCAPA);-   N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine    (abbreviation: 2PCABPhA);-   N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine    abbreviation: 2DPAPA);-   N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine    (abbreviation: 2DPABPhA);-   N-[9,10-bis(1,1′-biphenyl-2-yl)]-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine    (abbreviation: 2YGABPhA); N,N,9-triphenylanthracen-9-amine    (abbreviation: DPhAPhA); and the like. Moreover, as a light-emitting    material which exhibits yellowish light emission, rubrene,    5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation:    BPT), and the like can be used. As a light-emitting material which    exhibits reddish light emission,    N,N,′N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine    (abbreviation: p-mPhTD); 7,13-diphenyl-N,N′,N′    tetrakis(4-methylphenyl)acenaphtho[1,2-α]fluoranthene-3,10-d iamine    (abbreviation: p-mPhAFD), and the like can be used. Alternatively, a    phosphorescent material such as    bis[2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C^(3′)]iridium(III)acetylacetonate    (abbreviation: Ir(btp)₂(acac)) can be also used.

Note that the light-emitting layer may also have a structure in whichthe above highly light-emissive substance is dispersed in anothersubstance. Various substances can be used for the material in which thelight-emissive substance is dispersed. In particular, it is preferableto use a substance whose lowest unoccupied molecular orbital (LUMO)level is higher than that of the light-emissive substance and whosehighest occupied molecular orbital (HOMO) level is lower than that ofthe light-emissive substance.

Specifically, the following metal complexes can be used:tris(8-quinolinolato)aluminum(III) (abbreviation: Alq);tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃);bis(10-hydroxybenzo[h]-quinolinato)beryllium(II) (abbreviation: BeBq₂);bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(BAlq);bis(8-quinolinolato)zinc(II) (abbreviation: Znq);bis[2-(2-benzoxazolyl)phenolato]zinc(II) (Abbreviation: ZnPBO);bis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: Zn(BTZ)₂); andthe like. Further, the following heterocyclic compounds can be alsoused: 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole(abbreviation: PBD);1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7);3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ);2,2′,2″-(1,3,5-benzenetriyl)-tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI); bathophenanthroline (abbreviation: BPhen);bathocuproine (abbreviation: BCP); and the like. Furthermore, thefollowing condensed aromatic compounds can be also used:9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CZPA);3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: DPCZPA); 9,10-bis(3,5-diphenylphenyl)anthracene(abbreviation: DPPA); 9,10-di(2-naphthyl)anthracene (abbreviation: DNA);2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA);9,9′-bianthryl (abbreviation: BANT);9,9′-(stilben-3,3′-diyl)diphenanthrene (abbreviation: DPNS);9,9′-(stilben-4,4′-diyl)diphenanthrene (abbreviation: DPNS2);3,3′,3″-(benzene-1,3,5-triyl)tripyrene (abbreviation: TPB3);9,10-diphenylanthracene (abbreviation: DPAnth);6,12-dimethoxy-5,11-diphenylchrysene; and the like. Alternatively, thefollowing aromatic amine compounds can be used:N,N-dipheyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: CzA1PA); 4-(10-phenyl-9-anthryl)triphenylamine(abbreviation: DPhPA);N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA);N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}9H-carbazol-3-amine(abbreviation: PCAPBA);N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA); NPB (or α-NPD); TPD; DFLDPBi; BSPB; and thelike.

As a material in which the light-emissive substance is dispersed, pluralkinds of materials can be used. For example, a substance such as rubreneor like, which suppresses the crystallization of the light-emissivelayer, can be further added in order to suppress the crystallization ofthe light emissive substrate. In addition, NPB, Alq, or the like may befurther added in order to efficiently transfer energy to thelight-emissive substance.

When a structure in which a highly light-emissive substance is dispersedin another substance is employed, the crystallization of thelight-emitting layer 214 can be suppressed. Further, concentrationquenching which results from the high concentration of the highlylight-emissive substance can be also suppressed.

Compounds with a high molecular-weight can be also used for thelight-emitting layer 214. Specifically, as a light-emitting materialwhich exhibits bluish light emission, the following can be used:poly(9,9-dioctylfluorene-2,7-diyl) (abbreviation: POF);poly[(9,9-dioctylfluorene-2,7-diyl-co-(2,5-dimethoxybenzene-1,4-diyl)](abbreviation: PF-DMOP);poly{(9,9-dioctylfluorene-2,7-diyl)-co-[N,N′-di-(p-butylphenyl)-1,4-diaminobenzene]}(abbreviation: TAB-PFH); and the like. As a light-emitting materialwhich exhibits greenish light emission, the following can be used:poly(p-phenylenevinylene) (abbreviation: PPV);poly[(9,9-dihexylfluorene-2,7-diyl)-alt-co-(benzo[2,1,3]thiadiazol-4,7-diyl)](abbreviation: PFBT);poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)];and the like. As a light-emitting material which exhibits orangish toreddish light emission, the following can be used:poly[2-methoxy-5-(2′-ethylhexoxy)-1,4-phenylenevinylene] (abbreviation:MEH-PPV); poly(3-butylthiophene-2,5-diyl);poly{[9,9-dihexyl-2,7-bis(1-cyanovinylene)fluorenylene]-alt-co-[2,5-bis(N,N′-diphenylamino)-1,4-phenylene]};poly{[2-methoxy-5-(2-ethylhexyloxy)-1,4-bis(1-cyanovinylenephenylene)]-alt-co-[2,5-bis(N,N′-diphenylamino)-1,4-phenylene]}(abbreviation: CN-PPV-DPD); and the like.

The electron-transporting layer 215 is a layer containing a substancehaving a high electron-transporting property. For example, as organiccompounds with a low molecular-weight, the following metal complexes canbe used: tris(8-quinolinolato)aluminum(III) (abbreviation: Alq);tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃);bis(10-hydroxybenzo[h]-quinolinato)beryllium(II) (abbreviation: BeBq₂);bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(BAlq);bis(8-quinolinolato)zinc(II) (abbreviation: Znq);bis[2-(2-benzoxazolyl)phenolato]zinc(II)(Abbreviation: ZnPBO);bis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: Zn(BTZ)₂); andthe like. Further, the following heterocyclic compounds can be alsoused: 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole(abbreviation: PBD);1,3-bis[5-p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7);3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZOL);2,2′,2″-(1,3,5-benzenetriyl)-tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI); bathophenanthroline (abbreviation: BPhen);bathocuproine (abbreviation: BCP); and the like. The substancesdescribed here are substances with a mobility of 10⁻⁶ cm2/Vs or more.Note that substances other than the above substances may be also used asthe electron-transporting layer as long as the electron-transportingproperties thereof are higher than the hole-transporting propertiesthereof. Further, the electron-transporting layer is not limited to be asingle layer but may have a stacked structure in which two or morelayers formed of the above substances are stacked.

Further, the electron-transporting layer 215 can be also formed using acompound with a high molecular-weight. For example, the following can beused: poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridin-3,5-diyl)](abbreviation: PF-Py),poly[(9,9-dioctyllfluorene-2,7-diyl)-co-(2,2′-pyridin-6,6′-diyl)](abbreviation: PF-BPy), and the like.

The electron-injecting layer 216 is a layer containing a substance whichhas a high ability to promote electron-injection from the secondelectrode 204 to the EL layer 203. As a substance having such anability, alkali metals, alkaline earth metals, or compounds thereof canbe used such as lithium fluoride (LiF), cesium fluoride (CsF), andcalcium fluoride (CaF₂). For example, it is possible to use a layerformed of a substance having an electron-transporting property in whichan alkali earth metal, an alkaline earth metal, or a compound thereof ismixed, such as a mixture of Alq and magnesium (Mg). It is preferable touse the layer formed of a substance having an electron-transportingproperty in which an alkali earth metal, an alkaline earth metal, or acompound thereof is mixed is used because electrons can be efficientlyinjected from the second electrode 204.

The second electrode 204 is preferably formed using a substance with alow work function (specifically, 3.8 eV or less) such as metals, alloys,electrically conductive compounds, or a mixture thereof. Specificexamples of such a cathode material include an element belonging toGroup 1 or 2 of the periodic table, that is, alkali metals such alithium (Li) and cesium (Cs) and alkaline earth metals such as magnesium(Mg), calcium (Ca), and strontium (Sr), and alloys thereof (for example,MgAg and AlLi). Rare earth metals such as europium (Eu) and ytterbium(Yb), alloys thereof, and the like can be also used. A film made of analkali metal, an alkaline earth metal, or an alloy thereof can be formedby a vacuum deposition method. Further, a film formed of an alloy of analkali metal or an alkaline earth metal can be formed by a sputteringmethod. It is also possible to deposit a metal paste such as a silverpaste by an ink-jet method or the like.

When the electron-injecting layer 216 which is a layer having a functionof promoting electron injection is provided between the second electrode204 and the electron-transporting layer 215, the second electrode 204can be formed using various conductive materials such as Al, Ag, ITO,and ITO containing silicon or silicon oxide, regardless of their workfunctions. Such conductive materials can be deposited by a sputteringmethod, an ink-jet method, a spin coating method, or the like.

As a method forming the EL layer, various methods can be used regardlessof a dry process or a wet process. For example, a vacuum depositionmethod, an ink-jet method, a spin coating method, or the like can beused. Further, different deposition methods may be used for differentelectrodes or different layers.

For example, among the above materials, a compound with a highmolecular-weight may be selected to form the EL layer by a wet process.Alternatively, an organic compound with a low molecular-weight may beselected to form the EL layer by a wet process. Further, it is alsopossible to form the EL layer by depositing an organic compound with alow molecular-weight by using a dry process such as a vacuum depositionmethod.

Similarly, the electrodes can be formed by a wet process such as asol-gel process or by a wet process with a paste of a metal material.Alternatively, the electrodes can be formed by a dry process such as asputtering method or a vapor deposition method.

For example, in the case where the light-emitting element of the presentinvention is applied to a display device and its light-emitting layer isselectively deposited according to each color, the light-emitting layeris preferably formed by a wet process. When the light-emitting layer isformed by an ink-jet method, selective deposition of the light-emittinglayer for each color can be easily performed even when a large substrateis used.

In the light-emitting element of the present invention having the abovestructure, current flows due to a potential difference applied betweenthe first electrode 202 and the second electrode 204, whereby holes andelectrons are recombined in the EL layer 203 and light emission isobtained.

Light emission is extracted outside through one or both the firstelectrode 202 and the second electrode 204. Therefore, one or both thefirst electrode 202 and the second electrode 204 are light-transmittingelectrodes. When only the first electrode 202 is a light-transmittingelectrode, light emission is extracted from the substrate side throughthe first electrode 202 as illustrated in FIG. 3A. Meanwhile, when onlythe second electrode 204 is a light-transmitting electrode, lightemission is extracted from a side opposite to the substrate side throughthe second electrode 204 as illustrated in FIG. 3B. When both the firstelectrode 202 and the second electrode 204 are light-transmittingelectrodes, light emission is extracted from both the substrate side andthe side opposite to the substrate side through the first electrode 202and the second electrode 204 as illustrated in FIG. 3C.

Note that the structure of the layers provided between the firstelectrode 202 and the second electrode 204 is not limited to the abovestructure. Any structure other than the above structure may be employedas long as a light-emitting region in which holes and electrons arerecombined is provided apart from the first electrode 202 and the secondelectrode 204 in order to prevent quenching which occurs when thelight-emitting region is close to the electrodes, and as long as thelayer for controlling the carrier transport is provided.

That is, the structure of the layers provided between the firstelectrode 202 and the second electrode 204 is not particularly limited.Thus, layers which contain a substance having a highelectron-transporting property, a substance having a highhole-transporting property, a substance having a high electron-injectingproperty, a substance having a high hole-injecting property, and asubstance having a bipolar property (a substance with a highelectron-transporting property and a hole-transporting property), or thelike may be appropriately combined with the layer for controlling thecarrier transport and the light-emitting layer which are shown in thisembodiment mode.

Note that the layer for controlling the carrier transport shown in thisembodiment mode is a layer for controlling the transport of holes.Therefore, it is preferable to provide the layer for controlling thecarrier transport between the anode and the light-emitting layer.

When a layer is formed between the light-emitting layer 214 and thelayer 212 for controlling the carrier transport as illustrated in FIG.1A, that is, when the light-emitting layer 214 and the layer 212 forcontrolling the carrier transport are not in contact with each other,undesired interaction which occurs between the light-emitting layer 214and the layer 212 for controlling the carrier transport can besuppressed. Therefore, such a structure is preferable becausedeterioration of the light-emitting element can be suppressed.

As illustrated in FIG. 1B, a structure in which the light-emitting layer214 and the layer 212 for controlling the carrier transport are incontact with each other may be employed. In this structure, it ispreferable to use an organic compound, in which electrons are not easilyinjected and a band gap thereof is higher than that of an organiccompound having the highest weight percent among the organic compoundscontained in the light-emitting layer 214. In this case, thelight-emitting layer 214 and the layer 212 for controlling the carriertransport can be successively formed with the same mask. Thus, thisstructure is preferred in manufacturing a full-color display or the likewhere selective formation of the layer for controlling the carriertransport is required separately for each light-emitting element,because this structure facilitates the manufacture thereof.

The light-emitting elements illustrated in FIGS. 2A and 2B each have astructure in which the second electrode 204 which functions as acathode, the EL layer 203, and the first electrode 202 which functionsas an anode are sequentially stacked over the substrate 201. Thelight-emitting element illustrated in FIG. 2A has a structure in whichthe layers of the EL layer illustrated in FIG. 1A are stacked in areverse order, that is, the electron-injecting layer 216, theelectron-transporting layer 215, the light-emitting layer 214, thehole-transporting layer 213, the layer 212 for controlling the carriertransport, and the hole-injecting layer 211 are sequentially stacked.The light-emitting element illustrated in FIG. 2B has a structure inwhich the layers of the EL layer illustrated in FIG. 1B are stacked in areverse order, that is the electron-injecting layer 216, theelectron-transporting layer 215, the light-emitting layer 214, the layer212 for controlling the carrier transport, the hole-transporting layer213, and the hole-injecting layer 211 are sequentially stacked.

In this embodiment mode, the light-emitting element is formed over asubstrate made of glass, plastic, or the like. When a plurality of suchlight-emitting elements is formed over one substrate, a passive matrixtype light-emitting device can be manufactured. Alternatively, it isalso possible to form, for example, thin film transistors (TFTs) over asubstrate made of glass, plastic, or the like and form light-emittingelements on electrodes that are electrically connected to the TFTs.Accordingly, an active matrix type light-emitting device in which driveof the light-emitting elements is controlled with the TFTs can bemanufactured. Note that the structure of the TFTs is not particularlylimited. Either staggered TFTs or inversely staggered TFTs may beemployed. Further, a driver circuit formed on the TFT substrate may beformed from both N-channel and P-channel TFTs or from one of N-channeland P-channel TFTs. Furthermore, the crystallinity of a semiconductorfilm used for forming the TFTs is not specifically limited. Either anamorphous semiconductor film or a crystalline semiconductor film may beused.

A light-emitting element of the present invention includes a layer forcontrolling the carrier transport. The layer for controlling the carriertransport contains at least two kinds of substances. Therefore, byselecting the combination of substances and controlling the mixtureratio thereof, the thickness of the layer, or the like, carrier balancecan be precisely controlled.

Since the carrier balance can be controlled by selecting the combinationof substances and by controlling the mixture ratio thereof, thethickness of the layer, or the like, the carrier balance can be moreeasily controlled than in a conventional light-emitting element. Thatis, the transport of carriers can be controlled by controlling themixture ratio of the substances, the thickness of the layer, or thelike, without replacing the substances with a different one.

By improving the carrier balance with the layer for controlling thecarrier transport, the luminous efficiency of the light-emitting elementcan be improved.

Further, using the layer for controlling the carrier transport makes itpossible to prevent excessive holes from being injected and also toprevent holes from penetrating the light-emitting layer to reach theelectron-transporting layer or the electron-injecting layer. When holesreach the electron-transporting layer or the electron-injecting layer,the recombination probability in the light-emitting layer decreases (inother words, carrier balance is lost). This phenomenon results indecrease in luminous efficiency. Further, decrease in luminousefficiency over time is caused. That is, the lifetime of thelight-emitting element is decreased.

However, by using the layer for controlling the carrier transport asshown in this embodiment mode, it becomes possible to prevent excessiveholes from being injected and also to prevent holes from penetrating thelight-emitting layer to reach the electron-transporting layer or theelectron-injecting layer. Further, a decrease in luminous efficiencyover time can be suppressed. That is, a long-lifetime light-emittingelement can be obtained.

In the present invention, among the two or more kinds of substancescontained in the layer for controlling the carrier transport, the secondorganic compound which has a lower weight percent than the first organiccompound is used for controlling the carrier transport. Therefore, thetransport of carriers can be controlled with a component having thelowest concentration among the components contained in the layer forcontrolling the carrier transport. Thus, a long-lifetime light-emittingelement which does not easily deteriorate over time can be obtained.Namely, the carrier balance hardly changes compared with the case wherethe carrier balance is controlled with a single substance. For example,when the transport of carriers is controlled by a layer formed of asingle substance, the carrier balance of the whole layer is changed by apartial change in morphology, partial crystallization, or the like.Therefore, such a light-emitting element will readily deteriorate overtime. However, as shown in this embodiment mode, when the transport ofcarriers is controlled with a component having the lowest weight percentamong the components contained in the layer for controlling the carriertransport, it is possible to reduce the effects of morphological change,crystallization, aggregation, or the like, whereby deterioration overtime can be suppressed. Therefore, a long-lifetime light-emittingelement in which the luminous efficiency will not readily decrease overtime can be obtained.

As shown in this embodiment mode, a structure in which the layer forcontrolling the carrier transport is provided between the light-emittinglayer and the second electrode which functions as a cathode isparticularly effective for a light-emitting element which readily existsin the hole-excessive state upon driving. Since a light-emitting elementusing an organic compound tends to take the hole-excessive state in manycases, the present invention can be preferably applied to a number oflight-emitting elements using an organic compound.

Note that this embodiment mode can be combined with other embodimentmodes as appropriate.

EMBODIMENT MODE 2

This embodiment mode will describe a mode of a light-emitting element inwhich a plurality of light-emitting units according to the presentinvention are stacked (hereinafter, referred to as a stacked typeelement) with reference to FIG. 5. This light-emitting element is alight-emitting element including a plurality of light-emitting unitsbetween a first electrode and a second electrode.

In FIG. 5, a first light-emitting unit 511 and a second light-emittingunit 512 are stacked between a first electrode 501 and a secondelectrode 502. As to the first electrode 501 and the second electrode502, similar electrodes to those shown in Embodiment Mode 1 can beapplied. The first light-emitting unit 511 and the second light-emittingunit 512 may each have the same structure or different structure, and asimilar structure to that shown in Embodiment Mode 1 can be employed.

A charge generation layer 513 contains a composite material of anorganic compound and a metal oxide. The composite material of an organiccompound and a metal oxide is the composite material shown in EmbodimentMode 1, and includes an organic compound and a metal oxide such as V₂O₅,MoO₃, or WO₃. As the organic compound, various compounds such as anaromatic amine compound, a carbazole derivative, aromatic hydrocarbon,and a compound with a high molecular-weight (oligomer, dendrimer,polymer, or the like) can be used. As the organic compound, it ispreferable to use the organic compound which has a hole-transportingproperty with a hole mobility of 10⁻⁶ cm²/Vs or more. However, othersubstances than the materials described above may be also used as longas the substances have hole-transporting properties higher than theelectron transporting properties. The composite material of the organiccompound and the metal oxide can achieve low-voltage driving andlow-current driving because of the superior carrier-injecting propertyand carrier-transporting property.

Note that the charge generation layer 513 may be formed by combinationof a composite material of the organic compound and the metal oxide withother materials. For example, a layer containing a composite material ofthe organic compound and the metal oxide may be combined with a layercontaining one compound selected from substances having anelectron-donating property and a compound having a high electrontransporting property. Further, a layer containing a composite materialof the organic compound and the metal oxide may be combined with atransparent conductive film.

In any case, any kind of structure is acceptable as long as the chargegeneration layer 513 interposed between the first light-emitting unit511 and the second light-emitting unit 512 is capable of injectingelectrons into one of these light-emitting units and holes into theother when voltage is applied to the first electrode 501 and the secondelectrode 502.

Although this embodiment mode describes the light-emitting elementhaving two light-emitting units, the present invention can be similarlyapplied to a light-emitting element in which three or morelight-emitting units are stacked. When the charge generation layer isprovided between the pair of electrodes so as to partition the plurallight-emitting units like the light-emitting element of this embodimentmode, it is possible to provide a light-emitting element which exhibitsa high luminance at a low current density and a long lifetime. Thus,such a light-emitting element can be applied for illumination.

The light-emitting units can be designed to emit light having differentcolors from each other, thereby obtaining light emission of a desiredcolor from the whole light-emitting element. For example, in alight-emitting element having two light-emitting units, the emissioncolors of the first light-emitting unit and the second light-emittingunit are made complementary, so that a white-emissive light-emittingelement can be obtained. Note that the word “complementary” refers tothe color relationship in which an achromatic color is obtained whencolors are mixed. That is, mixing of complementarily colored light giveswhite light. The same relationship can be applied to a light-emittingelement having three light-emitting units. For example, when the firstlight-emitting unit emits red light, the second light-emitting unitemits green light, and the third light-emitting unit emits blue light,white light can be emitted from the light-emitting element.

Note that this embodiment mode can be combined with other embodimentmodes as appropriate.

EMBODIMENT MODE 3

This embodiment mode will describe a light-emitting device having alight-emitting element of the present invention.

This embodiment mode describes a light-emitting device having alight-emitting element of the present invention in a pixel portion, withreference to FIGS. 6A and 6B. FIG. 6A is a top view illustrating alight-emitting device while FIG. 6B is a cross-sectional view takenalong lines A-A′ and B-B′ of FIG. 6A. The light-emitting device includesa driver circuit portion (source-side driver circuit) 601, a pixelportion 602, and a driver circuit portion (gate-side driver circuit) 603which are illustrated with dotted lines. These units control lightemission of the light-emitting element. In addition, reference numeral604 denotes a sealing substrate; 605, a sealing material; and 607, aspace surrounded by the sealing material 605.

A leading wire 608 is to transmit a signal to be inputted to thesource-side driver circuit 601 and the gate-side driver circuit 603, andreceive a video signal, a clock signal, a start signal, a reset signal,and the like from an FPC (Flexible Printed Circuit) 609, which serves asan external input terminal. Although only the FPC is illustrated here,this FPC may be provided with a printed wiring board (PWB). Thelight-emitting device in this specification includes not only alight-emitting device itself but also a light-emitting device with anFPC or a PWB attached thereto.

Next, a cross-sectional structure is described with reference to FIG.6B. The driver circuit portion and the pixel portion are formed over anelement substrate 610. Here, the source-side driver circuit 601, whichis the driver circuit portion, and one pixel in the pixel portion 602are illustrated.

A CMOS circuit, which is a combination of an N-channel TFT 623 and aP-channel TFT 624, is formed as the source-side driver circuit 601. Thedriver circuit may be formed using various kinds of CMOS circuits, PMOScircuits, and NMOS circuits. Although a driver-integration type device,in which a driver circuit is formed over the same substrate as a pixelportion, is shown in this embodiment mode, a driver circuit is notnecessarily formed over the same substrate as a pixel portion and can beformed outside the substrate.

The pixel portion 602 has a plurality of pixels, each of which includesa switching TFT 611, a current-controlling TFT 612, and a firstelectrode 613 which is electrically connected to a drain of thecurrent-controlling TFT 612. Note that an insulator 614 is formed so asto cover an end portion of the first electrode 613. Here, a positivephotosensitive acrylic resin film is used for the insulator 614.

The insulator 614 is formed so as to have a curved surface havingcurvature at an upper end portion or a lower end portion thereof inorder to obtain favorable coverage. For example, in the case of using apositive photosensitive acrylic resin as a material for the insulator614, the insulator 614 is preferably formed so as to have a curvedsurface with a curvature radius (0.2 to 3 μm) only at the upper endportion thereof. Either a negative photoresist which becomes insolublein an etchant by light irradiation or a positive photoresist whichbecomes soluble in an etchant by light irradiation can be used for theinsulator 614.

An EL layer 616 and a second electrode 617 are formed over the firstelectrode 613. Various metals, alloys, electrically conductivecompounds, and mixture thereof can be used for a material for formingthe first electrode 613. When the first electrode 613 is used as ananode, it is particularly preferable to select a material with a highwork function (a work function of 4.0 eV or more) among such metals,alloys, electrically conductive compounds, and mixture thereof. Forexample, the first electrode 613 can be formed by using a single-layerfilm such as a film made of ITO containing silicon, a film made ofindium zinc oxide (IZO), a titanium nitride film, chromium film, atungsten film, a Zn film, or a Pt film; a stacked layer of a titaniumnitride film and a film containing aluminum as its main component; athree-layer structure of a titanium nitride film, a film containingaluminum as its main component, and a titanium nitride film; or thelike. When the first electrode 613 has a stacked layer structure, theelectrode 613 has low resistance as a wiring, giving a favorable ohmiccontact. Further, the first electrode 613 can function as an anode.

In addition, the EL layer 616 is formed by various methods such as avapor-deposition method using an evaporation mask, an ink-jet method,and a spin coating method. The EL layer 616 includes the light-emittinglayer shown in Embodiment Mode 1. As other materials for forming the ELlayer 616, a low molecular compound, oligomer, dendrimer, or a highmolecular compound may be also used. Further, not only organic compoundsbut also inorganic compounds can be used for the material for formingthe EL layer.

As a material for forming the second electrode 617, various metals,alloys, electrically conductive compounds, and mixture of them can beused. When the second electrode 617 is used as a cathode, it isparticularly preferable to select a material with a low work function (awork function of 3.8 eV or less) among such metals, alloys, electricallyconductive compounds, and mixture thereof. For example, elementsbelonging to Group 1 or 2 of the periodic table, that is, alkali metalssuch a lithium (Li) and cesium (Cs) and alkaline earth metals such asmagnesium (Mg), calcium (Ca), and strontium (Sr); alloys thereof (forexample, MgAg and AlLi); and the like can be given. In the case wherelight generated in the EL layer 616 is transmitted through the secondelectrode 617, the second electrode 617 may be also formed using astacked layer of a thin metal film and a transparent conductive film(for example, indium tin oxide (ITO), ITO containing silicon or siliconoxide, indium zinc oxide (IZO), or indium oxide containing tungstenoxide and zinc oxide (IWZO)).

By attaching the sealing substrate 604 to the element substrate 610 withthe sealing material 605, a light-emitting element 618 is provided inthe space 607 surrounded by the element substrate 610, the sealingsubstrate 604, and the sealing material 605. Note that the space 607 isfilled with a filler. Besides the case where the space is filled with aninert gas (for example, nitrogen, argon, or the like), there is also thecase where the space 607 is filled with the sealing material 605.

Note that an epoxy resin is preferably used for the sealing material605. Such material preferably allows as little moisture and oxygen aspossible to penetrate. As a material for forming the sealing substrate604, a glass substrate or a quartz substrate can be used as well as aplastic substrate made of FRP (Fiberglass-Reinforced Plastics), PVF(polyvinyl fluoride), polyester, acrylic, or the like.

By the above process, a light-emitting device having the light-emittingelement of the present invention can be obtained.

Although this embodiment mode has described the active matrix typelight-emitting device in which the driving of the light-emitting elementis controlled by a transistor as described above, the light-emittingdevice may be of a passive matrix type light-emitting device. FIG. 7A isa perspective view of a passive type light-emitting device manufacturedby applying the present invention. FIG. 7B is a cross-sectional viewtaken along a line X-Y of FIG. 7A. In each of FIGS. 7A and 7B, an ELlayer 955 is provided between an electrode 952 and an electrode 956 overa substrate 951. An end portion of the electrode 952 is covered with aninsulating layer 953. Then, a partition layer 954 is provided over theinsulating layer 953. A side wall of the partition layer 954 has such agradient that the distance between one side wall and the other side wallis shortened as approaching the substrate surface. That is, a crosssection of the partition layer 954 in a short-side direction istrapezoid-like, in which a bottom side (a side in a similar direction toa surface direction of the insulating layer 953, which is in contactwith the insulating layer 953) is shorter than an upper side (a side ina similar direction to the surface direction of the insulating layer953, which is not in contact with the insulating layer 953). In thisway, by providing the partition layer 954, a problem of defects in alight-emitting element due to electrostatic and the like can beprevented. Even in the case of the passive matrix type light-emittingdevice, a light-emitting device with high luminous efficiency can beobtained by including the light-emitting element of the presentinvention with high luminous efficiency.

The light-emitting device of the present invention has thelight-emitting element shown in Embodiment Mode 1 or 2. Thus, alight-emitting device with high luminous efficiency can be obtained.

Further, since the light-emitting device of the present invention hasthe light-emitting element with high luminous efficiency, alight-emitting device with low power consumption can be obtained.

Furthermore, since the light-emitting device of the present inventionhas the long-lifetime light-emitting element which is hardlydeteriorated, a long-lifetime light-emitting device can be obtained.

Note that this embodiment mode can be combined with other embodimentmodes as appropriate.

EMBODIMENT MODE 4

This embodiment mode will describe electronic devices of the presentinvention, which include the light-emitting device described inEmbodiment Mode 3 as part thereof. The electronic devices of the presentinvention each have the light-emitting element described in EmbodimentMode 1 or Embodiment Mode 2, and a display portion having a longlifetime.

As the electronic device manufactured by using the light-emitting deviceof the present invention, cameras such as video cameras or digitalcameras, goggle-type displays, navigation systems, audio reproducingdevices (such as car audios or audio components), computers, gamemachines, mobile information terminals (such as mobile computers,cellular phones, mobile game machines, or electronic books), imagereproducing devices equipped with a recording medium (specifically,devices equipped with a display device for reproducing a recordingmedium such as digital versatile disk (DVD) and displaying the image),and the like are given. Specific examples of these electronic devicesare illustrated in FIGS. 8A to 8D.

FIG. 8A illustrates a television device according to this embodimentmode, which includes a housing 9101, a support 9102, a display portion9103, a speaker portion 9104, a video input terminal 9105, and the like.In this television device, the display portion 9103 is formed byarranging similar light-emitting elements to those described inEmbodiment Modes 1 and 2 in a matrix form. The light-emitting elementshave a feature of high luminous efficiency and low power consumption.Further, there is a feature of a long lifetime. Since the displayportion 9103 formed using the light-emitting element also has a similarfeature, image quality is less deteriorated and low power consumption isachieved in this television device. Through such features, deteriorationcompensating function circuits and power supply circuits can be greatlyreduced in number or in size in the television device; therefore, areduction in the size and weight of the housing 9101 and the support9102 can be achieved. Since low power consumption, improvement in imagequality, and reduction in size and weight are achieved in the televisiondevice according to this embodiment mode, a product which is suitablefor a living environment can be provided.

FIG. 8B illustrates a computer according to this embodiment mode, whichincludes a main body 9201, a housing 9202, a display portion 9203, akeyboard 9204, an external connection port 9205, a pointing device 9206,and the like. In this computer, the display portion 9203 is formed byarranging similar light-emitting elements to those described inEmbodiment Modes 1 and 2 in a matrix form. The light-emitting elementshave a feature of high luminous efficiency and low power consumption.Further, there is a feature of a long lifetime. Since the displayportion 9203 formed using the light-emitting element also has thesimilar feature, image quality is less deteriorated and low powerconsumption is achieved in this computer. Through such features,deterioration compensating function circuits and power supply circuitscan be greatly reduced in number or in size in the computer. Therefore,a reduction in the size and weight of the main body 9201 and the housing9202 can be achieved. Since low power consumption, improvement in imagequality, and reduction in size and weight are achieved in the computeraccording to this embodiment mode, a product which is suitable for anenvironment can be provided. Further, the computer can be carried andthe computer having the display portion which has strong resistance toan impact from external when being carried can be provided.

FIG. 8C illustrates a cellular phone according to this embodiment mode,which includes a main body 9401, a housing 9402, a display portion 9403,an audio input portion 9404, an audio output portion 9405, operationkeys 9406, an external connection port 9407, an antenna 9408, and thelike. In this cellular phone, the display portion 9403 is formed byarranging similar light-emitting elements to those described inEmbodiment Modes 1 and 2 in a matrix form. The light-emitting elementshave a feature of high luminous efficiency and low power consumption.Further, there is a feature of a long lifetime. Since the displayportion 9403 formed using the light-emitting element also has a similarfeature, image quality is hardly deteriorated and low power consumptionis achieved in this cellular phone. Through such features, deteriorationcompensating function circuits and power supply circuits can be greatlyreduced in number or in size in the cellular phone. Therefore, areduction in the size and weight of the main body 9401 and the housing9402 can be achieved. Since low power consumption, improvement in imagequality, and reduction in size and weight are achieved in the cellularphone according to this embodiment mode, a product which is suitable forbeing carried can be provided. Further, a product having the displayportion which has strong resistance to an impact from external whenbeing carried can be provided.

FIG. 8D illustrates a camera according to this embodiment mode, whichincludes a main body 9501, a display portion 9502, a housing 9503, anexternal connection port 9504, a remote control receiving portion 9505,an image receiving portion 9506, a battery 9507, an audio input portion9508, operation keys 9509, an eyepiece portion 9510, and the like. Inthis camera, the display portion 9502 is formed by arranging similarlight-emitting elements to those described in Embodiment Modes 1 and 2in a matrix form. The light-emitting elements have a feature of highluminous efficiency and low power consumption. Further, there is afeature of a long lifetime. Since the display portion 9502 formed usingthe light-emitting element also has a similar feature, image quality ishardly deteriorated and low power consumption is achieved in thiscamera. Through such features, deterioration compensating functioncircuits and power supply circuits can be greatly reduced in number orin size in the camera. Therefore, a reduction in the size and weight ofthe main body 9501 can be achieved. Since low power consumption,improvement in image quality, and reduction in size and weight areachieved in the camera according to this embodiment mode, a productwhich is suitable for being carried can be provided. Further, a producthaving the display portion which has strong resistance to an impact fromexternal when being carried can be provided.

FIG. 9 illustrates an audio reproducing device according to thisembodiment mode, specifically a car audio, which includes a main body701, a display portion 702, and operation switches 703 and 704. Thedisplay portion 702 can be formed by using the light-emitting device (apassive matrix type or an active matrix type) shown in Embodiment Mode3. Further, the display portion 702 may be also formed using alight-emitting device of a segment type. In any case, through the use ofthe light-emitting element according to the present invention, a displayportion can be formed with the use of a vehicular power source (12 to 42V). The display portion which has a longer lifetime and is bright can beformed while low power consumption thereof is achieved. Furthermore,this embodiment mode has shown an in-car audio system; however, thelight-emitting device of the present invention may be also used forportable audio systems or audio systems for home use.

FIG. 10 illustrates a digital player as one example of the audioreproducing device. The digital player illustrated in FIG. 10 includes amain body 710, a display portion 711, a memory portion 712, an operationportion 713, a pair of earphones 714, and the like. Note that a pair ofheadphones or a pair of wireless earphones can be used instead of thepair of earphones 714. The display portion 711 can be formed by usingthe light-emitting device (a passive matrix type or an active matrixtype) shown in Embodiment Mode 3. Further, the display portion 711 maybe also formed using a light-emitting device of a segment type. In anycase, through the use of the light-emitting element according to thepresent invention, display can be also performed with a secondarybattery (a nickel-hydrogen battery or the like). The display portion 711which has a longer lifetime and is bright can be formed while low powerconsumption thereof is achieved. As the memory portion 712, a hard diskor a nonvolatile memory is used. For example, a NAND type nonvolatilememory with a recording capacity of 20 to 200 gigabytes (GBs) is used,and the operation portion 713 is operated, whereby an image or a sound(for example, music) can be recorded and reproduced. The displayportions 704 and 711 display white characters on a black background sothat power consumption can be suppressed. This is particularly effectivefor a portable audio system.

As thus described, an application range of the light-emitting devicewhich is manufactured by applying the present invention is quite wide,and this light-emitting device can be applied to electronic devices ofevery field. By applying the present invention, an electronic devicehaving a display portion which consumes low power and has highreliability can be manufactured.

Moreover, the light-emitting device to which the present invention isapplied has a light-emitting element with high luminous efficiency, andthe light-emitting device can be also used as a lighting device. Onemode of using, as a lighting device, the light-emitting element to whichthe present invention is applied is described with reference to FIG. 11.

FIG. 11 illustrates a liquid crystal display device in which thelight-emitting device of the present invention is used as a backlight,as an example of the electronic device using the light-emitting deviceof the present invention. The liquid crystal display device illustratedin FIG. 11 includes a housing 901, a liquid crystal layer 902, abacklight 903, and a housing 904, in which the liquid crystal layer 902is connected to a driver IC 905. The light-emitting device of thepresent invention is used for the backlight 903, and current is suppliedto the backlight 903 through a terminal 906.

When the light-emitting device of the present invention is used as thebacklight of the liquid crystal display device, a backlight with highluminous efficiency can be obtained. Moreover, a long-lifetime backlightcan be also obtained. Further, since the light-emitting device of thepresent invention is a lighting device of surface light emission and theenlargement of the light-emitting device is possible, the backlight canbe made larger and the liquid crystal display device can also have alarger area. Furthermore, since the light-emitting device of the presentinvention is thin and consumes less power, reduction in thickness andpower consumption of the display device is possible.

FIG. 12 illustrates an example in which the light-emitting device towhich the present invention is applied is used as a desk lamp, which isa lighting device. The desk lamp illustrated in FIG. 12 includes ahousing 2001 and a light source 2002. The light-emitting device of thepresent invention is used as the light source 2002. Since thelight-emitting device of the present invention has a long lifetime, thedesk lamp can also have a long lifetime.

FIG. 13 illustrates an example of using the light-emitting device towhich the present invention is applied as a lighting device 3001 in theroom. Since the light-emitting device of the present invention can beenlarged, the light-emitting device can be used as a large-area lightingdevice. Moreover, since the light-emitting device of the presentinvention has a long lifetime, the lighting device can also have a longlifetime. Thus, a television device 3002 according to the presentinvention similar to the television device described with reference toFIG. 8A can be installed in the room using, as the lighting device 3001,the light-emitting device to which the present invention is applied sothat pubic broadcasting and movies can be enjoyed. In such a case, sinceboth the television device and the lighting device have long lifetimes,it is unnecessary to often buy new lighting device or television device(that is, the number of replacing is small) and it is possible to reducea load to the environment.

Note that this embodiment mode can be combined with other embodimentmodes as appropriate.

EMBODIMENT 1

This embodiment will specifically describe a light-emitting element ofthe present invention with reference to FIG. 14. Note that alight-emitting element 1, a light-emitting element 2, and a comparativelight-emitting element 3 were formed over the same substrate. Structuralformulas of organic compounds used in Embodiment 1 are shown below.

(Manufacture of Light-Emitting Element 1)

First, a film of indium tin oxide containing silicon oxide was formedover a glass substrate 2201 by a sputtering method to form a firstelectrode 2202. Note that the thickness was 110 nm and the electrodearea was 2 mm×2 mm.

Next, the substrate on which the first electrode 2202 was formed wasfixed to a substrate holder that was provided in a vacuum evaporationapparatus, such that the surface on which the first electrode 2202 wasformed came to the lower side. After the pressure of the vacuumevaporation apparatus was reduced to be approximately 10⁻⁴ Pa,4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum (VI) oxide were co-evaporated on the first electrode 2202 toform a layer 2211 containing the composite material. The thickness was30 nm, and the evaporation rate was controlled so that the weight ratioof NPB to molybdenum (VI) oxide could be 4:1 (=NPB: molybdenum oxide).Note that the co-evaporation method is an evaporation method in whichevaporations from a plurality of evaporation sources are performed atthe same time in one treatment chamber.

Next, a layer 2212 for controlling the carrier transport was formed overthe layer 2211 containing a composite material. The layer 2212 forcontrolling the carrier transport was formed by co-evaporating4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) and1,3-bis[5-p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7) at a thickness of 10 nm. Here, the evaporationrate was controlled so that the weight ratio of NPB to OXD-7 could be1:0.05 (=NPB:OXD-7).

Next, by an evaporation method using resistance heating, a film of4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) wasformed so as to have a thickness of 20 nm, giving a hole-transportinglayer 2213.

Next, a light-emitting layer 2214 was formed over the hole-transportinglayer 2213. 9-[4-(10-phenyl-9-antryl)phenyl]-9H-carbazole (abbreviation:CzPA) andN,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S) were co-evaporated to form the light-emittinglayer 2214 with a thickness of 30 nm. Here, the evaporation rate wascontrolled so that the weight ratio of CzPA to YGA2S could be 1:0.04(=CzPA:YGA2S).

After that, an electron-transporting layer 2215 was formed over thelight-emitting layer 2214 by an evaporation method using resistanceheating. First, tris(8-quinolinolato)aluminum(III) (abbreviation: Alq)was formed over the light-emitting layer 2214 so as to have a thicknessof 20 nm to form a first electron-transporting layer 2231. Then,bathophenanthroline (abbreviation: BPhen) was formed over the firstelectron-transporting layer 2231 so as to have a thickness of 10 nm toform a second electron-transporting layer 2232.

Next, by an evaporation method using resistance heating, a film oflithium fluoride (LiF) was formed over the electron-transporting layer2215 so as to have a thickness of 1 nm, giving an electron-injectinglayer 2216.

Finally, a film of aluminum was formed so as to have a thickness of 200nm by an evaporation method using resistance heating to form a secondelectrode 2204. In this manner, a light-emitting element 1 wasmanufactured.

The light-emitting element 1 of the present invention obtained throughthe above process was put into a glove box containing a nitrogen so thatthe light-emitting element was sealed in order not to be exposed toatmospheric air. Then, the operating characteristics of thelight-emitting element were measured. Note that the measurement wasperformed at a room temperature (atmosphere kept at 25° C.).

(Manufacture of Light-Emitting Element 2)

A light-emitting element 2 was manufactured in a similar manner to thelight-emitting element 1 except that the weight ratio of NPB to OXD-7 inthe layer 2212 for controlling the carrier transport was 1:0.1(=NPB:OXD-7).

As to the light-emitting element 2 of the present invention, thelight-emitting element was also put into a glove box containing anitrogen so that the light-emitting element was sealed in order not tobe exposed to atmospheric air, in a similar manner to the light-emittingelement 1. Then, the operating characteristics of the light-emittingelement were measured. The measurement was performed at a roomtemperature (atmosphere kept at 25° C.).

(Manufacture of Comparative Light-Emitting Element 3)

Next, for comparison, the comparative light-emitting element 3 having astructure in which the layer 2212 for controlling the carrier transportin the above light-emitting element 1 and light-emitting element 2 isnot provided was formed. The manufacturing method is described below.

First, a film of indium tin oxide containing silicon oxide was formedover a glass substrate by a sputtering method to form a first electrode.Note that the thickness was 110 nm and the electrode area was 2 mm×2 mm.

Next, the substrate on which the first electrode was formed was fixed toa substrate holder that was provided in a vacuum evaporation apparatus,such that the surface on which the first electrode was formed came tothe lower side. After the pressure of the vacuum evaporation apparatuswas reduced to be approximately 10⁻⁴ Pa,4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum (VI) oxide were co-evaporated on the first electrode to forma layer containing a composite material. The thickness was 30 nm, andthe evaporation rate was controlled so that the weight ratio of NPB tomolybdenum (VI) oxide could be 4:1 (=NPB:molybdenum oxide).

Next, by an evaporation method using resistance heating, a film of4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) wasformed over the layer containing a composite material so as to have athickness of 30 nm to form a hole-transporting layer.

Next, a light-emitting layer was formed over the hole-transportinglayer. 9-[4-(10-phenyl-9-antryl)phenyl]-9H-carbazole (abbreviation CzPA)andN,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S) were co-evaporated to form the light-emittinglayer with a thickness of 30 nm. Here, the evaporation rate wascontrolled so that the weight ratio of CzPA to YGA2S could be 1:0.04(=CzPA:YGA2S).

After that, an electron-transporting layer was formed over thelight-emitting layer by an evaporation method using resistance heating.First, tris(8-quinolinolato)aluminum(III) (abbreviation: Alq) was formedover the light-emitting layer so as to have a thickness of 20 nm to forma first electron-transporting layer. Then, bathophenanthroline(abbreviation: BPhen) was formed over the first electron-transportinglayer so as to have a thickness of 10 nm to form a secondelectron-transporting layer.

Next, by an evaporation method using resistance heating, a film oflithium fluoride (LiF) was formed over the electron-transporting layerso as to have a thickness of 1 nm to form an electron-injecting layer.

Finally, a film of aluminum was formed so as to have a thickness of 200nm by an evaporation method using resistance heating to form a secondelectrode. In this manner, a comparative light-emitting element 3 wasmanufactured.

As to the comparative light-emitting element 3 obtained through theabove process, the light-emitting element was also put into a glove boxcontaining a nitrogen so that the light-emitting element was sealed inorder not to be exposed to atmospheric air, in a similar manner to thelight-emitting element 1. Then, the operating characteristics of thelight-emitting element were measured. Note that the measurement wasperformed at a room temperature (atmosphere kept at 25° C.).

FIG. 15 illustrates the current density vs. luminance characteristics ofthe light-emitting element 1, the light-emitting element 2, and thecomparative light-emitting 3. FIG. 16 illustrates the voltage vs.luminance characteristics thereof. FIG. 17 illustrates the luminance vs.current efficiency characteristics thereof. FIG. 18 illustrates theemission spectra thereof obtained at a current supply of 1 mA.

The emission color of the light-emitting element 1 was located at theCIE chromaticity coordinates of (x=0.16, y=0.17) at the luminance of 950cd/m², and blue emission which derives from YGA2S was obtained. Inaddition, a current efficiency, a voltage, a current density, and apower efficiency of the light-emitting element 1 at the luminance of 950cd/m² were 4.6 cd/A, 5.4 V, 20.7 mA/cm², and 2.6 lm/W, respectively.

The emission color of the light-emitting element 2 was located at theCIE chromaticity coordinates of (x=0.16, y=0.17) at the luminance of 950cd/m², and blue emission which derives from YGA2S was obtained. Inaddition, a current efficiency, a voltage, a current density, and apower efficiency of the light-emitting element 2 at the luminance of 950cd/m² were 6.0 cd/A, 5.4 V, 15.7 mA/cm², and 3.5 lm/W, respectively.

The emission color of the comparative light-emitting element 3 waslocated at the CIE chromaticity coordinates of (x=0.16, y=0.16) at theluminance of 850 cd/m², and blue emission which derives from YGA2S wasobtained. In addition, a current efficiency, a voltage, a currentdensity, and a power efficiency of the comparative light-emittingelement 3 at the luminance of 850 cd/m² were 3.6 cd/A, 5.2 V, 23.5mA/cm², and 2.2 lm/W, respectively.

As described above, it is found that high current efficiency is obtainedin the light-emitting element 1 and the light-emitting element 2 in eachof which the layer for controlling the carrier transport of the presentinvention is provided, as compared with the comparative light-emittingelement 3 without the layer for controlling the carrier transport.Further, since the driving voltage of the light-emitting element 1 andthe light-emitting element 2 is not much different from that of thecomparative light-emitting element 3 in which the layer for controllingthe carrier transport is not provided. Therefore, the light-emittingelement 1 and the light-emitting element 2 show high power efficiency,which is contributed by their high current efficiency. Thus, it is foundthat the light-emitting element of the present invention consumes lowpower.

FIG. 19 illustrates the result of a continuous lighting test in whichthe light-emitting element 1 and the comparative light-emitting element3 were continuously driven at constant current with the initialluminance set at 1000 cd/m². The vertical axis indicates the relativeluminance (normalized luminance) on the assumption that 1000 cd/m² is100%. As apparent from FIG. 19, it is found that the light-emittingelement 1 in which the layer for controlling the carrier transport ofthe present invention is provided has almost the same lifetime as thecomparative light-emitting element 3 in which the layer for controllingthe carrier transport is not provided. This means that the layer forcontrolling the carrier transport does not affect the lifetime butimproves the current efficiency of the light-emitting element.

The dipole moment of OXD-7 used in the light-emitting element 1 and thelight-emitting element 2 was calculated. First, the structure of aground state of OXD-7 was optimized by density functional theory (DFT)at a level of B3LYP/6-311(d,p). The dipole moment of OXD-7 with anoptimized structure was calculated to be 3.78 debye. The accuracy ofcalculation of the DFT is higher than that of a Hartree-Fock (HF) methodwhich does not consider electron correlation. In addition, thecalculation cost of the DFT is lower than that of a method ofperturbation (MP) which has the same level accuracy of calculation asthe DFT. Therefore, the DFT was employed in this calculation. Thecalculation was performed using a high performance computer (HPC)(manufactured by SGI Japan, Ltd., Altix3700 DX).

The oxidation characteristics of NPB and OXD-7, which were used in thelayer for controlling the carrier transport in the light-emittingelement 1 and the light-emitting element 2 manufactured in thisembodiment, were evaluated by using the cyclic voltammetry (CV)measurement. The ionization potentials of NPB and OXD-7 were obtainedfrom the CV measurement results. Note that an electrochemical analyzer(ALS model 600A or 600C, product of BAS Inc.) was used for themeasurement.

In the CV measurement, dehydrated N,N-dimethylformamide (DMF, product ofSigma-Aldrich Inc., 99.8%, catalog No. 22705-6) was used for a solvent,and tetra-n-butylammonium perchlorate (n-Bu₄NClO₄, product of TokyoChemical Industry Co., Ltd., catalog No. T0836), which is a supportingelectrolyte, was dissolved in DMF at the concentration oftetra-n-butylammonium perchlorate of 100 mmol/L. The object compound tobe measured was also dissolved in the DMF-solution containing n-Bu₄NClO₄at a concentration of 10 mmol/L. A platinum electrode (a PTE platinumelectrode, product of BAS Inc.) was used as a working electrode; aplatinum electrode (a VC-3 Pt counter electrode (5 cm), product of BASInc.) was used as an auxiliary electrode; and an Ag/Ag⁺ electrode (anRE5 nonaqueous solvent reference electrode, product of BAS Inc.) wasused as a reference electrode. The CV measurement was conducted at roomtemperature (20 to 25° C.).

(Calculation of the Potential Energy of the Reference Electrode withRespect to the Vacuum Level)

First, potential energy (eV) of the reference electrode (Ag/Ag⁺electrode) used in this embodiment with respect to the vacuum level wascalculated. That is, the Fermi level of the Ag/Ag⁺ electrode wascalculated. It is known that the oxidation potential of ferrocene inmethanol is +0.610 V with respect to a standard hydrogen electrode(Reference: Christian R. Goldsmith et al., J. Am. Chem. Soc., Vol. 124,No. 1, pp. 83-96, 2002). The oxidation potential of ferrocene inmethanol measured by using the reference electrode used in thisembodiment was found to be +0.20 V vs. Ag/Ag⁺. Therefore, it wasconfirmed that the potential energy of the reference electrode used inthis embodiment was lower than that of the standard hydrogen electrodeby 0.41 eV.

Here, it is also known that the potential energy of the standardhydrogen electrode with respect to the vacuum level is −4.44 eV(Reference: Toshihiro Ohnishi and Tamami Koyama, High molecular ELmaterial, Kyoritsu Shuppan, pp. 64-67). Accordingly, the potentialenergy of the reference electrode used in this embodiment with respectto the vacuum level was determined to be −4.44-0.41=−4.85 eV.

(Measurement Example 1: Oxd-7)

In this measurement example, the oxidation characteristics of OXD-7 wereevaluated by cyclic voltammetry (CV) measurement. The scan rate was setat 0.1 V/sec. FIG. 38 illustrates the measurement result. Note that themeasurement of the oxidation characteristics was performed by the stepsof: scanning the potential of the working electrode with respect to thereference electrode in the ranges of −0.47 V to −1.60 V, and then 1.60 Vto −0.47 V.

As illustrated in FIG. 38, it is found that a peak which corresponds tothe oxidation of OXD-7 did not appear even when scanning was performedup to 1.0 V Even if there were a peak which indicates oxidation at avoltage greater than or equal to 1.0 V, the peak could not be observeddue to the influence of flow of a large amount of current. Therefore, itcan be concluded from this data that the oxidation potential of OXD-7 isgreater than or equal to 1.0 V. Since the potential energy of thereference electrode used in this measurement example with respect to thevacuum level is −4.85 eV, an oxidation potential of 1.0 V in the CVmeasurement corresponds to an ionization potential of −(−4.85-1.0)=5.85eV. Therefore, it was found that the ionization potential of OXD-7 wasat least greater than or equal to 5.8 eV. Thus, it is found that OXD-7can be preferably used for the layer for controlling the carriertransport of the present invention.

(Measurement Example 2: NPB)

In this measurement example, the oxidation characteristics of NPB wereevaluated by cyclic voltammetry (CV) measurement. The scan rate was setat 0.1 V/sec. FIG. 37 illustrates the measurement result. Themeasurement of the oxidation characteristics was performed by the stepsof: scanning the potential of the working electrode with respect to thereference electrode in the ranges of −0.20 V to 0.80 V, and then 0.80 Vto −0.20 V.

As illustrated in FIG. 37, an oxidation peak potential E_(pa) appearedat 0.45 V. Therefore, a difference in oxidation peak potential betweenNPB and OXD-7 measured in the Measurement Example 1 is greater than orequal to 0.55 V. Thus, a difference between the oxidation peak potentialof NPB and that of OXD-7 is greater than or equal to 0.5 V, which meansthat a difference between an ionization potential of NPB and that ofOXD-7 is at least greater than or equal to 0.5 eV.

As shown above, since the dipole moment and the ionization potential ofOXD-7 used for the light-emitting element 1 and the light-emittingelement 2 are greater than or equal to 2.0 debye and greater than orequal to 5.8 eV, respectively, OXD-7 can be preferably used for thelayer for controlling the carrier transport. That is, the layercontaining NPB which is an organic compounds having a hole-transportingproperty and OXD-7 functions as the layer for controlling the carriertransport.

Accordingly, it was revealed that carrier balance is improved byapplying the present invention and thus a light-emitting element withhigh luminous efficiency can be obtained. Further, it was confirmed thata light-emitting element with low power consumption can be obtained.

EMBODIMENT 2

This embodiment will specifically describe a light-emitting element ofthe present invention with reference to FIG. 20. Note that alight-emitting element 4 and a light-emitting element 5 manufactured inEmbodiment 2 were formed over the same substrate.

(Manufacture of Light-Emitting Element 4)

First, a film of indium tin oxide containing silicon oxide was formedover a glass substrate 2201 by a sputtering method, and a firstelectrode 2202 was formed. Note that the thickness was 110 nm and theelectrode area was 2 mm×2 mm.

Next, the substrate on which the first electrode 2202 was formed wasfixed to a substrate holder that was provided in a vacuum evaporationapparatus, such that the surface on which the first electrode 2202 wasformed came to the lower side. After the pressure of the vacuumevaporation apparatus was reduced to be approximately 10⁻⁴ Pa,4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum (VI) oxide were co-evaporated on the first electrode 2202 toform a layer 2211 containing a composite material. The thickness was 50nm, and the evaporation rate was controlled so that the weight ratio ofNPB to molybdenum (VI) oxide could be 4:1 (=NPB:molybdenum oxide).

Next, a layer 2212 for controlling the carrier transport was formed overthe layer 2211 containing a composite material. The layer 2212 forcontrolling the carrier transport was formed by co-evaporating4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) and1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7) at a thickness of 10 nm. Here, the evaporationrate was controlled so that the weight ratio of NPB to OXD-7 could be1:0.05 (=NPB:OXD-7).

Next, a light-emitting layer 2214 was formed over the layer 2212 forcontrolling the carrier transport.9-[4-(10-phenyl-9-antryl)phenyl]-9H-carbazole (abbreviation CZPA) andN,N″-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S) were co-evaporated to form the light-emittinglayer 2214 with a thickness of 30 nm. Here, the evaporation rate wascontrolled so that the weight ratio of CzPA to YGA2S could be 1:0.04(=CzPA:YGA2S).

After that, an electron-transporting layer 2215 was formed over thelight-emitting layer 2214 by an evaporation method using resistanceheating. First, tris(8-quinolinolato)aluminum(III) (abbreviation: Alq)was formed over the light-emitting layer 2214 so as to have a thicknessof 20 nm to form a first electron-transporting layer 2231. Then,bathophenanthroline (abbreviation: BPhen) was formed over the firstelectron-transporting layer 2231 so as to have a thickness of 10 nm toform a second electron-transporting layer 2232.

Next, by an evaporation method using resistance heating, a film oflithium fluoride (LiF) was formed over the electron-transporting layer2215 so as to have a thickness of 1 nm to form an electron-injectinglayer 2216.

Finally, a film of aluminum was formed so as to have a thickness of 200nm by an evaporation method using resistance heating to form a secondelectrode 2204. In this manner, a light-emitting element 4 wasmanufactured.

The light-emitting element 4 of the present invention obtained throughthe above process was put into a glove box containing a nitrogen so thatthe light-emitting element was sealed in order not to be exposed toatmospheric air. Then, the operating characteristics of thelight-emitting element were measured. Note that the measurement wasperformed at a room temperature (atmosphere kept at 25° C.).

(Manufacture of Light-Emitting Element 5)

A light-emitting element 5 was manufactured in a similar manner to thelight-emitting element 4 except that the weight ratio of NPB to OXD-7 inthe layer 2212 for controlling the carrier transport was 1:0.1(=NPB:OXD-7).

As to the light-emitting element 5 of the present invention, thelight-emitting element was also put into a glove box containing anitrogen so that the light-emitting element was sealed in order not tobe exposed to atmospheric air, in a similar manner to the light-emittingelement 4. Then, the operating characteristics of the light-emittingelement were measured. Note that the measurement was performed at a roomtemperature (atmosphere kept at 25° C.).

FIG. 21 illustrates the current density vs. luminance characteristics ofthe light-emitting element 4 and the light-emitting element 5. FIG. 22illustrates the voltage vs. luminance characteristics thereof. FIG. 23illustrates the luminance vs. current efficiency characteristicsthereof. FIG. 24 illustrates the emission spectra thereof obtained at acurrent supply of 1 mA.

The emission color of the light-emitting element 4 was located at theCIE chromaticity coordinates of (x=0.16, y=0.16) at the luminance of 980cd/m², and blue emission which derives from YGA2S was obtained. Inaddition, a current efficiency, a voltage, a current density, and apower efficiency of the light-emitting element 4 at the luminance of 980cd/m² were 4.5 cd/A, 4.8 V, 21.7 mA/cm², and 3.0 lm/W, respectively.

The emission color of the light-emitting element 5 was located at theCIE chromaticity coordinates of (x=0.16, y=0.17) at the luminance of 940cd/m², and blue emission which derives from YGA2S was obtained. Inaddition, a current efficiency, a voltage, a current density, and apower efficiency of the light-emitting element 5 at the luminance of 940cd/m² were 5.7 cd/A, 4.6 V, 16.5 mA/cm², and 3.9 lm/W, respectively.

As described above, it is found that high current efficiency is obtainedin the light-emitting element 4 and the light-emitting element 5 in eachof which the layer for controlling the carrier transport of the presentinvention is provided. Further, high power efficiency is obtained in thelight-emitting element 4 and the light-emitting element 5, which iscontributed by their high current efficiency. Thus, it can be concludedthat the light-emitting element of the present invention consumes lowpower.

Accordingly, it was found that carrier balance is improved by applyingthe present invention and thus a light-emitting element with highluminous efficiency can be obtained. Further, it was revealed that alight-emitting element with low power consumption can be obtained.

The dipole moment of OXD-7 used for the light-emitting element 4 and thelight-emitting element 5 is 3.78 debye as calculated in Embodiment 1,and the ionization potential of OXD-7 is greater than or equal to 5.8eV. The difference in ionization potential between NPB and OXD-7 isgreater than or equal to 0.5 eV.

Thus, it can be concluded that OXD-7 used for the light-emitting element4 and the light-emitting element 5 can be preferably used for the layerfor controlling the carrier transport. That is, the layer containing NPBwhich is an organic compounds having a hole-transporting property andOXD-7 functions as the layer for controlling the carrier transport.

The structure in which the light-emitting layer and the layer forcontrolling the carrier transport are in contact with each other wasshown in this embodiment, and it was found that, even in this structure,the layer containing NPB and OXD-7 functions as the layer forcontrolling the carrier transport, resulting in the formation of alight-emitting element with high luminous efficiency.

EMBODIMENT 3

This embodiment will specifically describe a light-emitting element ofthe present invention with reference to FIG. 14. Note that alight-emitting element 6, a light-emitting element 7, and a comparativelight-emitting element 8 manufactured in Embodiment 3 were formed overthe same substrate. A structural formula of an organic compound used inEmbodiment 3 is shown below.

(Manufacture of Light-Emitting Element 6)

First, a film of indium tin oxide containing silicon oxide was formedover a glass substrate 2201 by a sputtering method, and a firstelectrode 2202 was formed. Note that the thickness was 110 nm and theelectrode area was 2 mm×2 mm.

Next, the substrate on which the first electrode 2202 was formed wasfixed to a substrate holder that was provided in a vacuum evaporationapparatus, such that the surface on which the first electrode 2202 wasformed came to the lower side. After the pressure of the vacuumevaporation apparatus was reduced to be approximately 10⁻⁴ Pa,4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum (VI) oxide were co-evaporated on the first electrode 2202 toform a layer 2211 containing a composite material. The thickness was 30nm, and the evaporation rate was controlled so that the weight ratio ofNPB to molybdenum (VI) oxide could be 4:1 (=NPB:molybdenum oxide).

Next, a layer 2212 for controlling the carrier transport was formed overthe layer 2211 containing a composite material. The layer 2212 forcontrolling the carrier transport was formed by depositing4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) and3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZOL) at a thickness of 10 nm by a co-evaporationmethod. Here, the evaporation rate was controlled so that the weightratio of NPB to TAZ01 could be 1:0.05 (=NPB:TAZOL).

Next, by an evaporation method using resistance heating, a film of4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) wasformed so as to have a thickness of 20 nm to form a hole-transportinglayer 2213.

Next, a light-emitting layer 2214 was formed over the hole-transportinglayer 2213. 9-[4-(10-phenyl-9-antryl)phenyl]-9H-carbazole (abbreviation:CzPA) andN,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S) were co-evaporated to form the light-emittinglayer 2214 with a thickness of 30 nm. Here, the evaporation rate wascontrolled so that the weight ratio of CzPA to YGA2S could be 1:0.04(=CzPA:YGA2S).

After that, an electron-transporting layer 2215 was formed over thelight-emitting layer 2214 by an evaporation method using resistanceheating. First, tris(8-quinolinolato)aluminum(III) (abbreviation: Alq)was formed over the light-emitting layer 2214 so as to have a thicknessof 20 nm to form a first electron-transporting layer 2231. Then,bathophenanthroline (abbreviation: BPhen) was formed over the firstelectron-transporting layer 2231 so as to have a thickness of 10 nm toform a second electron-transporting layer 2232.

Next, by an evaporation method using resistance heating, a film oflithium fluoride (LiF) was formed over the electron-transporting layer2215 so as to have a thickness of 1 nm to form an electron-injectinglayer 2216.

Finally, a film of aluminum was formed so as to have a thickness of 200nm by an evaporation method using resistance heating to form a secondelectrode 2204. In this manner, a light-emitting element 6 wasmanufactured.

The light-emitting element 6 of the present invention obtained throughthe above process was put into a glove box containing a nitrogen so thatthe light-emitting element was sealed without exposing to atmosphericair. Then, the operating characteristics of the light-emitting elementwere measured. Note that the measurement was performed at a roomtemperature (atmosphere kept at 25° C.).

(Manufacture of Light-Emitting Element 7)

A light-emitting element 7 was manufactured in a similar manner to thelight-emitting element 6 except that the weight ratio of NPB to TAZ01 inthe layer 2212 for controlling the carrier transport was 1:0.1(=NPB:TAZ01).

As to the light-emitting element 7 of the present invention, thelight-emitting element was also put into a glove box containing anitrogen so that the light-emitting element was sealed in order not tobe exposed to atmospheric air, in a similar manner to the light-emittingelement 6. Then, the operating characteristics of the light-emittingelement were measured. Note that the measurement was performed at a roomtemperature (atmosphere kept at 25° C.).

(Manufacture of Comparative Light-Emitting Element 8)

Next, for comparison, the comparative light-emitting element 8 having astructure in which the layer 2212 for controlling the carrier transportin the above light-emitting element 6 and light-emitting 7 is notprovided was formed. The manufacturing method is described below.

First, a film of indium tin oxide containing silicon oxide was formedover a glass substrate by a sputtering method, and a first electrode wasformed. Note that the thickness was 110 nm and the electrode area was 2mm×2 mm.

Next, the substrate on which the first electrode was formed was fixed toa substrate holder that was provided in a vacuum evaporation apparatus,such that the surface on which the first electrode was formed came tothe lower side. After the pressure of the vacuum evaporation apparatuswas reduced to be approximately 10⁻⁴ Pa,4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum (VI) oxide were co-evaporated on the first electrode,resulting in the formation of the layer containing a composite material.The thickness was 30 nm, and the evaporation rate was controlled so thatthe weight ratio of NPB to molybdenum (VI) oxide could be 4:1(=NPB:molybdenum oxide).

Next, by an evaporation method using resistance heating, a film of4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) wasformed over the layer containing a composite material so as to have athickness of 30 nm to form a hole-transporting layer.

Next, a light-emitting layer was formed over the hole-transportinglayer. 9-[4-(10-phenyl-9-antryl)phenyl]-9H-carbazole (abbreviation:CzPA) andN,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S) were co-evaporated to form the light-emittinglayer with a thickness of 30 nm. Here, the evaporation rate wascontrolled so that the weight ratio of CzPA to YGA2S could be 1:0.04(=CzPA:YGA2S).

After that, an electron-transporting layer was formed over thelight-emitting layer by an evaporation method using resistance heating.First, tris(8-quinolinolato)aluminum(III) (abbreviation: Alq) was formedover the light-emitting layer so as to have a thickness of 20 nm to forma first electron-transporting layer. Then, bathophenanthroline(abbreviation: BPhen) was formed over the first electron-transportinglayer so as to have a thickness of 10 nm to form a secondelectron-transporting layer.

Next, by an evaporation method using resistance heating, a film oflithium fluoride (LiF) was formed over the electron-transporting layerso as to have a thickness of 1 nm to form an electron-injecting layer.

Finally, a film of aluminum was formed so as to have a thickness of 200nm by an evaporation method using resistance heating to form a secondelectrode. In this manner, a comparative light-emitting element 8 wasmanufactured.

As to the comparative light-emitting element 8 obtained through theabove process, the light-emitting element was also put into a glove boxcontaining a nitrogen so that the light-emitting element was sealed inorder not to be exposed to atmospheric air, in a similar manner to thelight-emitting element 6. Then, the operating characteristics of thelight-emitting element were measured. Note that the measurement wasperformed at a room temperature (atmosphere kept at 25° C.).

FIG. 25 illustrates the current density vs. luminance characteristics ofthe light-emitting element 6, the light-emitting element 7, and thecomparative light-emitting 8. FIG. 26 illustrates the voltage vs.luminance characteristics thereof. FIG. 27 illustrates the luminance vs.current efficiency characteristics thereof. FIG. 28 illustrates theemission spectra thereof obtained at a current supply of 1 mA.

The emission color of the light-emitting element 6 was located at theCIE chromaticity coordinates of (x=0.16, y=0.15) at the luminance of 810cd/m², and blue emission which derives from YGA2S was obtained. Inaddition, a current efficiency, a voltage, a current density, and apower efficiency of the light-emitting element 6 at the luminance of 810cd/m² were 3.3 cd/A, 5.8 V, 24.2 mA/cm², and 1.8 μm/W, respectively.

The emission color of the light-emitting element 7 was located at theCIE chromaticity coordinates of (x=0.16, y=0.15) at the luminance of 930cd/m², and blue emission which derives from YGA2S was obtained. Inaddition, a current efficiency, a voltage, a current density, and apower efficiency of the light-emitting element 7 at the luminance of 930cd/m² were 3.9 cd/A, 5.8 V, 23.5 mA/cm², and 2.2 lm/W, respectively.

The emission color of the comparative light-emitting element 8 waslocated at the CIE chromaticity coordinates of (x=0.16, y=0.15) at theluminance of 1040 cd/m², and blue emission which derives from YGA2S wasobtained. In addition, a current efficiency, a voltage, a currentdensity, and a power efficiency of the comparative light-emittingelement 8 at the luminance of 1040 cd/m² were 2.8 cd/A, 5.8 V, 36.6mA/cm², and 1.5 ml/W, respectively.

As described above, it is found that high current efficiency is obtainedin the light-emitting element 6 and the light-emitting element 7 inwhich the layer for controlling the carrier transport of the presentinvention is provided, as compared with the comparative light-emittingelement 8 in which the layer for controlling the carrier transport isnot provided. Further, since the driving voltage of the light-emittingelement 6 and the light-emitting element 7 is not much different fromthat of the comparative light-emitting element 8 without the layer forcontrolling the carrier transport, the light-emitting elements 6 and 7show high power efficiency, originating from their high currentefficiency. Thus, it is found that the light-emitting element of thepresent invention consumes low power.

The dipole moment of TAZOL used in the light-emitting element 6 and thelight-emitting element 7 was calculated. First, the structure of aground state of TAZ01 was optimized by density functional theory (DFT)at a level of B3LYP/6-311(d,p). The dipole moment of TAZOL with anoptimized structure was calculated to be 5.87 debye. The calculation wascarried out in a similar way shown in Embodiment 1.

The oxidation characteristics of TAZ01 which was used for the layer forcontrolling the carrier transport in the light-emitting element 6 andthe light-emitting element 7 were evaluated by cyclic voltammetry (CV)measurement. The measurement was performed under the same conditions asthat of Measurement Example 1 in Embodiment 1.

(Measurement Example 3: TAZ01)

In this measurement example, the oxidation characteristics of TAZ01 wereevaluated by cyclic voltammetry (CV) measurement. The scan rate was setat 0.1 V/sec. FIG. 39 illustrates the measurement result. Themeasurement of the oxidation characteristics was performed by the stepsof: scanning the potential of the working electrode with respect to thereference electrode in the ranges of −0.47 V to −1.50 V, and then 1.50 Vto −0.47 V.

As illustrated in FIG. 39, a peak which indicates oxidation of TAZ01 didnot appear even when scanning was performed at least up to 1.0 V.Further, even if there were a peak which indicates oxidation at avoltage greater than or equal to 1.0 V, the peak could not be observeddue to the influence of flow of a large amount of current. That is, itcan be concluded from this data that the oxidation potential of TAZOL isgreater than or equal to 1.0 V. Since the potential energy of thereference electrode used in this measurement example with respect to thevacuum level is −4.85 eV, an oxidation potential of 1.0 V in the CVmeasurement corresponds to be an ionization potential of −(−4.85−1.0)=5.85 eV. Therefore, it was found that the ionization potential ofTAZ01 is at least greater than or equal to 5.8 eV, which means thatTAZ01 can be preferably used for the layer for controlling the carriertransport of the present invention.

As shown in Measurement Example 2 in Embodiment 1, an oxidation peakpotential E_(pa) of NPB is 0.45 V, and a difference in oxidation peakpotential of TAZ01 measured in Measurement Example 3 is greater than orequal to 0.55 V. Therefore, a difference between the oxidation peakpotential of NPB and the oxidation peak potential of TAZ01 is greaterthan or equal to 0.5 V. Accordingly, a difference between an ionizationpotential of NPB and that of TAZ01 is at least greater than or equal to0.5 eV.

Thus, it was confirmed that, since a dipole moment and an ionizationpotential of TAZ01 used for the light-emitting element 6 and thelight-emitting element 7 are larger than or equal to 2.0 debye andgreater than or equal to 5.8 eV, respectively, TAZ01 can be preferablyused for the layer for controlling the carrier transport. That is, itwas found that a layer containing NPB and TAZ01 which are organiccompounds having a hole-transporting property functions as the layer forcontrolling the carrier transport. In particular, since a dipole momentof TAZ01 is large, TAZ01 is preferable as a second organic compoundwhich is used for the layer for controlling the carrier transport.

Accordingly, it was revealed that carrier balance is improved byapplying the present invention and thus a light-emitting element withhigh luminous efficiency can be obtained. Further, it was found that alight-emitting element with low power consumption can be obtained.

EMBODIMENT 4

This embodiment will specifically describe a light-emitting element ofthe present invention with reference to FIG. 20. Note that alight-emitting element 9 and a light-emitting element 10 manufactured inEmbodiment 4 were formed over the same substrate.

(Manufacture of Light-Emitting Element 9)

First, a film of indium tin oxide containing silicon oxide was formedover a glass substrate 2201 by a sputtering method, and a firstelectrode 2202 was formed. Note that the thickness was 110 nm and theelectrode area was 2 mm×2 mm.

Next, the substrate on which the first electrode 2202 was formed wasfixed to a substrate holder that was provided in a vacuum evaporationapparatus, such that the surface on which the first electrode 2202 wasformed came to the lower side. After the pressure of the vacuumevaporation apparatus was reduced to be approximately 10⁻⁴ Pa,4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum (VI) oxide were co-evaporated on the first electrode 2202 toform a layer 2211 containing a composite material. The thickness was 50nm, and the evaporation rate was controlled so that the weight ratio ofNPB to molybdenum (VI) oxide could be 4:1 (=NPB:molybdenum oxide).

Next, a layer 2212 for controlling the carrier transport was formed overthe layer 2211 containing a composite material. The layer 2212 forcontrolling the carrier transport was formed by depositing4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) and3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZOL) at a thickness of 10 nm by a co-evaporationmethod. Here, the evaporation rate was controlled so that the weightratio of NPB to TAZOL could be 1:0.05 (=NPB:TAZ01).

Next, a light-emitting layer 2214 was formed over the layer 2212 forcontrolling the carrier transport.9-[4-(10-phenyl-9-antryl)phenyl]-9H-carbazole (abbreviation CzPA) andN,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S) were co-evaporated to form the light-emittinglayer 2214 with a thickness of 30 nm. Here, the evaporation rate wascontrolled so that the weight ratio of CzPA to YGA2S could be 1:0.04(=CzPA:YGA2S).

After that, an electron-transporting layer 2215 was formed over thelight-emitting layer 2214 by an evaporation method using resistanceheating. First, tris(8-quinolinolato)aluminum(III) (abbreviation: Alq)was formed over the light-emitting layer 2214 so as to have a thicknessof 20 nm to form a first electron-transporting layer 2231. Then,bathophenanthroline (abbreviation: BPhen) was formed over the firstelectron-transporting layer 2231 so as to have a thickness of 10 nm toform a second electron-transporting layer 2232.

Next, by an evaporation method using resistance heating, a film oflithium fluoride (LiF) was formed over the electron-transporting layer2215 so as to have a thickness of 1 nm to form an electron-injectinglayer 2216.

Finally, a film of aluminum was formed so as to have a thickness of 200nm by an evaporation method using resistance heating to form a secondelectrode 2204. In this manner, a light-emitting element 9 wasmanufactured.

The light-emitting element 9 of the present invention obtained throughthe above process was put into a glove box containing a nitrogen so thatthe light-emitting element was sealed in order not to be exposed toatmospheric air. Then, the operating characteristics of thelight-emitting element were measured. Note that the measurement wasperformed at a room temperature (atmosphere kept at 25° C.).

(Manufacture of Light-Emitting Element 10)

A light-emitting element 10 was manufactured in a similar manner to thelight-emitting element 9 except that the weight ratio of NPB to TAZ01 inthe layer 2212 for controlling the carrier transport was 1:0.1(=NPB:TAZ01).

As to the light-emitting element 10 of the present invention, thelight-emitting element was also put into a glove box containing anitrogen so that the light-emitting element was sealed in order not tobe exposed to atmospheric air, in a similar manner to the light-emittingelement 9. Then, the operating characteristics of the light-emittingelement were measured. Note that the measurement was performed at a roomtemperature (atmosphere kept at 25° C.).

FIG. 29 illustrates the current density vs. luminance characteristics ofthe light-emitting element 9 and the light-emitting element 10. FIG. 30illustrates the voltage vs. luminance characteristics thereof. FIG. 31illustrates the luminance vs. current efficiency characteristicsthereof. FIG. 32 illustrates the emission spectra thereof obtained at acurrent supply of 1 mA.

The emission color of the light-emitting element 9 was located at theCIE chromaticity coordinates of (x=0.16, y=0.15) at the luminance of 930cd/m², and blue emission which derives from YGA2S was obtained. Inaddition, a current efficiency, a voltage, a current density, and apower efficiency of the light-emitting element 9 at the luminance of 930cd/m² were 3.8 cd/A, 5.2 V, 24.4 mA/cm², and 2.3 lm/W, respectively.

The emission color of the light-emitting element 10 was located at theCIE chromaticity coordinates of (x=0.16, y=0.15) at the luminance of 930cd/m², and blue emission which derives from YGA2S was obtained. Inaddition, a current efficiency, a voltage, a current density, and apower efficiency of the light-emitting element 10 at the luminance of930 cd/m² were 4.8 cd/A, 5.0 V, 19.5 mA/cm², and 3.0 lm/W, respectively.

As described above, it is found that high current efficiency is obtainedin the light-emitting element 9 and the light-emitting element 10 ineach of which the layer for controlling the carrier transport of thepresent invention is provided. The light-emitting element 9 and thelight-emitting element 10 have high power efficiency due to the highcurrent efficiency thereof. Thus, it is found that the light-emittingelement of the present invention consumes low power.

Accordingly, it can be concluded that carrier balance is improved byapplying the present invention and thus a light-emitting element withhigh luminous efficiency can be obtained. Further, it was found that alight-emitting element with low power consumption can be obtained.

The dipole moment of TAZ01 used for the light-emitting element 9 and thelight-emitting element 10 is 5.87 debye as calculated in Embodiment 3and an ionization potential of TAZOL is greater than or equal to 5.8 eV.The difference between the ionization potential of NPB and that of TAZ01is greater than or equal to 0.5 eV.

Thus, it is found that TAZOL used for the light-emitting element 9 andthe light-emitting element 10 can be preferably used for the layer forcontrolling the carrier transport. That is, it is confirmed that a layercontaining NPB which is an organic compound having a hole-transportingproperty and TAZ01 functions as the layer for controlling the carriertransport.

The structure in which the light-emitting layer and the layer forcontrolling the carrier transport are in contact with each other isshown in this embodiment, and it is found that, even in this structure,the layer containing NPB and TAZ01 functions as the layer forcontrolling the carrier transport, so that high luminous efficiency canbe obtained.

EMBODIMENT 5

This embodiment will specifically describe a light-emitting element ofthe present invention with reference to FIG. 14. Note that alight-emitting element 11, a light-emitting element 12, and acomparative light-emitting element 13 manufactured in Embodiment 5 wereformed over the same substrate. A structural formula of an organiccompound used in Embodiment 5 is shown below.

(Manufacture of Light-Emitting Element 11)

First, a film of indium tin oxide containing silicon oxide was formedover a glass substrate 2201 by a sputtering method to form a firstelectrode 2202. Note that the thickness was 110 nm and the electrodearea was 2 mm×2 mm.

Next, the substrate on which the first electrode 2202 was formed wasfixed to a substrate holder that was provided in a vacuum evaporationapparatus, such that the surface on which the first electrode 2202 wasformed came to the lower side. After the pressure of the vacuumevaporation apparatus was reduced to be approximately 10⁻⁴ Pa,4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum (VI) oxide were co-evaporated on the first electrode 2202,giving the layer 2211 containing a composite material. The thickness was30 nm, and the evaporation rate was controlled so that the weight ratioof NPB to molybdenum (VI) oxide could be 4:1 (=NPB:molybdenum oxide).

Next, a layer 2212 for controlling the carrier transport was formed overthe layer 2211 containing a composite material. The layer 2212 forcontrolling the carrier transport was formed by depositing4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andbathocuproine (abbreviation: BCP) at a thickness of 10 nm by aco-evaporation method. Here, the evaporation rate was controlled so thatthe weight ratio of NPB to BCP could be 1:0.05 (=NPB: BCP).

Next, by an evaporation method using resistance heating, a film of4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) wasformed so as to have a thickness of 20 nm to form a hole-transportinglayer 2213.

Next, a light-emitting layer 2214 was formed over the hole-transportinglayer 2213. 9-[4-(10-phenyl-9-antryl)phenyl]-9H-carbazole (abbreviation:CzPA) andN,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S) were co-evaporated to form the light-emittinglayer 2214 with a thickness of 30 mm. Here, the evaporation rate wascontrolled so that the weight ratio of CzPA to YGA2S could be 1:0.04(=CzPA:YGA2S).

After that, an electron-transporting layer 2215 was formed over thelight-emitting layer 2214 by an evaporation method using resistanceheating. First, tris(8-quinolinolato)aluminum(III) (abbreviation: Alq)was formed over the light-emitting layer 2214 so as to have a thicknessof 20 nm to form a first electron-transporting layer 2231. Then,bathophenanthroline (abbreviation: BPhen) was formed over the firstelectron-transporting layer 2231 so as to have a thickness of 10 nm toform a second electron-transporting layer 2232.

Next, by an evaporation method using resistance heating, a film oflithium fluoride (LiF) was formed over the electron-transporting layer2215 so as to have a thickness of 1 nm to form an electron-injectinglayer 2216.

Finally, a film of aluminum was formed so as to have a thickness of 200nm by an evaporation method using resistance heating to form a secondelectrode 2204. In this manner, a light-emitting element 11 wasmanufactured.

The light-emitting element 11 of the present invention obtained throughthe above process was put into a glove box containing a nitrogen so thatthe light-emitting element was sealed in order not to be exposed toatmospheric air. Then, the operating characteristics of thelight-emitting element were measured. Note that the measurement wasperformed at a room temperature (atmosphere kept at 25° C.).

(Manufacture of Light-Emitting Element 12)

A light-emitting element 12 was manufactured in a similar manner to thelight-emitting element 11 except that the weight ratio of NPB to BCP inthe layer 2212 for controlling the carrier transport was 1:0.1(=NPB:BCP).

As to the light-emitting element 12 of the present invention, thelight-emitting element was also put into a glove box containing anitrogen so that the light-emitting element was sealed in order not tobe exposed to atmospheric air, in a similar manner to the light-emittingelement 11. Then, the operating characteristics of the light-emittingelement were measured. Note that the measurement was performed at a roomtemperature (atmosphere kept at 25° C.).

The light-emitting element 12 of the present invention obtained throughthe above process was put into a glove box containing a nitrogen so thatthe light-emitting element was sealed in order not to be exposed toatmospheric air. Then, the operating characteristics of thelight-emitting element were measured. Note that the measurement wasperformed at a room temperature (atmosphere kept at 25° C.).

(Manufacture of Comparative Light-Emitting Element 13)

Next, for comparison, a comparative light-emitting element 13 having astructure in which the layer 2212 for controlling the carrier transportin the above light-emitting element 11 and light-emitting 12 is notprovided was formed. The manufacturing method is described below.

First, a film of indium tin oxide containing silicon oxide was formedover a glass substrate by a sputtering method to form a first electrode.Note that the thickness was 110 nm and the electrode area was 2 mm×2 mm.

Next, the substrate on which the first electrode was formed was fixed toa substrate holder that was provided in a vacuum evaporation apparatus,such that the surface on which the first electrode was formed came tothe lower side. After the pressure of the vacuum evaporation apparatuswas reduced to be approximately 10⁻⁴ Pa,4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum (VI) oxide were co-evaporated on the first electrode to forma layer containing a composite material. The thickness was 30 nm, andthe evaporation rate was controlled so that the weight ratio of NPB tomolybdenum (VI) oxide could be 4:1 (=NPB:molybdenum oxide).

Next, by an evaporation method using resistance heating, a film of4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) wasformed over the layer containing a composite material so as to have athickness of 30 nm to form a hole-transporting layer.

Next, a light-emitting layer was formed over the hole-transportinglayer. 9-[4-(10-phenyl-9-antryl)phenyl]-9H-carbazole (abbreviation:CzPA) andN,N-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S) were co-evaporated to form the light-emittinglayer at a thickness of 30 nm. Here, the evaporation rate was controlledso that the weight ratio of CzPA to YGA2S could be 1:0.04 (=CzPA:YGA2S).

After that, an electron-transporting layer was formed over thelight-emitting layer by an evaporation method using resistance heating.First, tris(8-quinolinolato)aluminum(III) (abbreviation: Alq) was formedover the light-emitting layer so as to have a thickness of 20 nm to forma first electron-transporting layer. Then, bathophenanthroline(abbreviation: BPhen) was formed over the first electron-transportinglayer so as to have a thickness of 10 nm to form a secondelectron-transporting layer.

Next, by an evaporation method using resistance heating, a film oflithium fluoride (LiF) was formed over the electron-transporting layerso as to have a thickness of 1 nm to form an electron-injecting layer.

Finally, a film of aluminum was formed so as to have a thickness of 200nm by an evaporation method using resistance heating to form a secondelectrode. In this manner, a comparative light-emitting element 13 wasmanufactured.

The light-emitting element 13 of the present invention obtained throughthe above process was put into a glove box containing a nitrogen so thatthe light-emitting element was sealed in order not to be exposed toatmospheric air. Then, the operating characteristics of thelight-emitting element were measured. Note that the measurement wasperformed at a room temperature (atmosphere kept at 25° C.).

FIG. 33 illustrates the current density vs. luminance characteristics ofthe light-emitting element 11, the light-emitting element 12, and thecomparative light-emitting 13. FIG. 34 illustrates the voltage vs.luminance characteristics thereof. FIG. 35 illustrates the luminance vs.current efficiency characteristics thereof. FIG. 36 illustrates theemission spectra thereof obtained at a current supply of 1 mA.

The emission color of the light-emitting element 11 was located at theCIE chromaticity coordinates of (x=0.16, y=0.18) at the luminance of1130 cd/m², and blue emission which derives from YGA2S was obtained. Inaddition, a current efficiency, a voltage, a current density, and apower efficiency of the light-emitting element 11 at the luminance of1130 cd/m² were 5.2 cd/A, 5.4 V, 21.9 mA/cm², and 3.0 lm/W,respectively.

The emission color of the light-emitting element 12 was located at theCIE chromaticity coordinates of (x=0.16, y=0.18) at the luminance of1080 cd/m², and blue emission which derives from YGA2S was obtained. Inaddition, a current efficiency, a voltage, a current density, and apower efficiency of the light-emitting element 12 at the luminance of1080 cd/m² were 5.9 cd/A, 5.4 V, 18.4 mA/cm², and 3.4 lm/W,respectively.

The emission color of the comparative light-emitting element 13 waslocated at the CIE chromaticity coordinates of (x=0.16, y=0.17) at theluminance of 740 cd/m², and blue emission which derives from YGA2S wasobtained. In addition, a current efficiency, a voltage, a currentdensity, and a power efficiency of the comparative light-emittingelement 13 at the luminance of 740 cd/m² were 3.6 cd/A, 4.8 V, 20.4mA/cm², and 2.3 lm/W, respectively.

As described above, it is found that high current efficiency is obtainedin the light-emitting element 11 and the light-emitting element 12 inwhich the layer for controlling the carrier transport of the presentinvention is provided, as compared with the comparative light-emittingelement 13 without the layer for controlling the carrier transport.Since the driving voltage of the light-emitting element 11 and thelight-emitting element 12 is not much different from that of thecomparative light-emitting element 13 without the layer for controllingthe carrier transport, high power efficiency is obtained, resulting fromthe high current efficiency thereof. Thus, it is found that thelight-emitting element of the present invention consumes low power.

A dipole moment of BCP used in the light-emitting element 11 and thelight-emitting element 12 was calculated. First, the structure of aground state of BCP was optimized by density functional theory (DFT) ata level of B3LYP/6-311(d,p). The dipole moment of the optimized BCP wascalculated to be 2.90 debye. The calculation was carried out in asimilar way to that shown in Embodiment 1.

The oxidation characteristics of BCP which was used for the layer forcontrolling the carrier transport in the light-emitting element 11 andthe light-emitting element 12 manufactured in this embodiment wereevaluated by cyclic voltammetry (CV) measurement. The measurement wasperformed under the same conditions as that of Measurement Example 1 inEmbodiment 1.

(Measurement Example 4: Bcp)

In this measurement example, the oxidation characteristics of BCP wereevaluated by cyclic voltammetry (CV) measurement. The scan rate was setat 0.1 V/sec. FIG. 40 illustrates the measurement result. Themeasurement of the oxidation characteristics was performed by the stepsof: scanning the potential of the working electrode with respect to thereference electrode in the ranges of −0.20 V to −1.60 V, and then 1.60 Vto −0.20 V.

As illustrated in FIG. 40, a peak which corresponds to the oxidation ofBCP did not appear even when scanning was performed at least up to 1.0V. Further, even if there were a peak which indicates oxidation at avoltage greater than or equal to 1.0 V, the peak could not be observeddue to the influence of flow of a large amount of current. This meansthat the oxidation potential of BCP is greater than or equal to 1.0 V.Since the potential energy of the reference electrode used in thismeasurement example with respect to the vacuum level is −4.85 eV, anoxidation potential of 1.0 V in the CV measurement corresponds to anionization potential of −(−4.85 −1.0)=5.85 eV. Therefore, it can beconcluded that the ionization potential of BCP is greater than or equalto 5.8 eV. Thus, it was confirmed that BCP can be preferably used forthe layer for controlling the carrier transport of the presentinvention.

As shown in Measurement Example 2 in Embodiment 1, an oxidation peakpotential E_(pa) of NPB is 0.45 V, and a difference from an oxidationpeak potential of BCP measured in Measurement Example 4 is greater thanor equal to 0.55 V. Therefore, a difference between the oxidation peakpotential of NPB and that of BCP is greater than or equal to 0.5 VAccordingly, the difference between the ionization potential of NPB andthat of BCP is greater than or equal to 0.5 eV.

Thus, it is found that, since the dipole moment and the ionizationpotential of BCP used for the light-emitting element 11 and thelight-emitting element 12 are greater than or equal to 2.0 debye andgreater than or equal to 5.8 eV, respectively, BCP can be preferablyused for the layer for controlling the carrier transport. Namely, alayer containing NPB which is an organic compound having ahole-transporting property and BCP functions as the layer forcontrolling the carrier transport.

These data lead to a conclusion that carrier balance can be improved byapplying the present invention and thus a light-emitting element withhigh luminous efficiency can be obtained. Further, it was found that alight-emitting element with low power consumption can be obtained.

The present application is based on Japanese Patent Application serialno. 2006-327662 filed with Japan Patent Office on Dec., 4, in 2006, theentire contents of which are hereby incorporated by reference.

1. A light-emitting element comprising: a light-emitting layer and afirst layer between an anode and a cathode, wherein the first layer isprovided between the light-emitting layer and the anode, wherein thefirst layer contains a first organic compound and a hole-blockingorganic compound, wherein a weight percent of the first organic compoundis larger than a weight percent of the hole-blocking organic compound,and wherein the first organic compound has a hole transport property. 2.The light-emitting element according to claim 1, wherein an ionizationpotential of the hole-blocking organic compound is larger than anionization potential of the first organic compound, and wherein adifference between the ionization potential of the first second organiccompound and the ionization potential of the hole-blocking organiccompound is larger than or equal to 0.5 eV.
 3. The light-emittingelement according to claim 1, wherein an ionization potential of thehole-blocking organic compound is larger than or equal to 5.8 eV.
 4. Thelight-emitting element according to claim 1, wherein the weight percentof the hole-blocking organic compound in the first layer is greater thanor equal to 1 weight percent and less than or equal to 20 weightpercent.
 5. The light-emitting element according to claim 1, wherein athickness of the first layer is greater than or equal to 1 nm and lessthan or equal to 20 nm.
 6. A light-emitting element comprising: alight-emitting layer and a first layer between an anode and a cathode,wherein the first layer is provided between the light-emitting layer andthe anode, wherein the first layer contains a first organic compound anda second organic compound, wherein a weight percent of the first organiccompound is larger than a weight percent of the second organic compound,wherein the first organic compound has a hole transport property, andwherein a dipole moment of the second organic compound is larger than orequal to 2.0 debye.
 7. The light-emitting element according to claim 6,wherein an ionization potential of the second organic compound is largerthan an ionization potential of the first organic compound, and whereina difference between the ionization potential of the first secondorganic compound and the ionization potential of the second organiccompound is larger than or equal to 0.5 eV.
 8. The light-emittingelement according to claim 6, wherein an ionization potential of thesecond organic compound is larger than or equal to 5.8 eV.
 9. Thelight-emitting element according to claim 6, wherein the weight percentof the second organic compound in the first layer is greater than orequal to 1 weight percent and less than or equal to 20 weight percent.10. The light-emitting element according to claim 6, wherein a thicknessof the first layer is greater than or equal to 1 nm and less than orequal to 20 nm.
 11. A light-emitting element comprising: alight-emitting layer and a first layer between an anode and a cathode,wherein the first layer is provided between the light-emitting layer andthe anode, wherein the first layer contains a first organic compound anda second organic compound, wherein a weight percent of the first organiccompound is larger than a weight percent of the second organic compound,wherein the first organic compound has a hole transport property, andwherein the second organic compound has a heterocycle.
 12. Thelight-emitting element according to claim 11, wherein the second organiccompound is selected from an oxadiazole derivative, a triazolederivative, and phenanthroline derivative.
 13. The light-emittingelement according to claim 11, wherein an ionization potential of thesecond organic compound is larger than an ionization potential of thefirst organic compound, and wherein a difference between the ionizationpotential of the first second organic compound and the ionizationpotential of the second organic compound is greater than or equal to 0.5eV.
 14. The light-emitting element according to claim 11, wherein anionization potential of the second organic compound is larger than orequal to 5.8 eV.
 15. The light-emitting element according to claim 11,wherein the weight percent of the second organic compound in the firstlayer is greater than or equal to 1 weight percent and less than orequal to 20 weight percent.
 16. The light-emitting element according toclaim 11, wherein a thickness of the first layer is greater than orequal to 1 nm and less than or equal to 20 nm.
 17. A light-emittingapparatus having a light-emitting element, the light-emitting elementcomprising a light-emitting layer and a first layer between an anode anda cathode, wherein the first layer is provided between thelight-emitting layer and the anode, wherein the first layer contains afirst organic compound and a hole-blocking organic compound, wherein aweight percent of the first organic compound is larger than a weightpercent of the hole-blocking organic compound, and wherein the firstorganic compound has a hole transport property.
 18. The light-emittingapparatus according to claim 17, wherein an ionization potential of thehole-blocking organic compound is larger than an ionization potential ofthe first organic compound, and wherein a difference between theionization potential of the first second organic compound and theionization potential of the hole-blocking organic compound is largerthan or equal to 0.5 eV.
 19. The light-emitting apparatus according toclaim 17, wherein an ionization potential of the hole-blocking organiccompound is larger than or equal to 5.8 eV.
 20. The light-emittingapparatus according to claim 17, wherein a dipole moment of thehole-blocking organic compound is larger than or equal to 2.0 debye. 21.The light-emitting apparatus according to claim 17, wherein thelight-emitting apparatus is a lighting device.
 22. A light-emittingapparatus having a light-emitting element, the light-emitting elementcomprising a light-emitting layer and a first layer between an anode anda cathode, wherein the first layer is provided between thelight-emitting layer and the anode, wherein the first layer contains afirst organic compound and a second organic compound, wherein a weightpercent of the first organic compound is larger than a weight percent ofthe second organic compound, wherein the first organic compound has ahole transport property, and wherein the second organic compound has aheterocycle.
 23. The light-emitting apparatus according to claim 22,wherein an ionization potential of the second organic compound is largerthan an ionization potential of the first organic compound, and whereina difference between the ionization potential of the first secondorganic compound and the ionization potential of the second organiccompound is greater than or equal to 0.5 eV.
 24. The light-emittingapparatus according to claim 22, wherein the second organic compound isselected from an oxadiazole derivative, a triazole derivative, andphenanthroline derivative.
 25. The light-emitting apparatus according toclaim 22, wherein the light-emitting apparatus is a lighting device.